A History of Science, Volume 3
by
Henry Smith Williams
Part 5 out of 6
the galvanic battery had a decided effect upon certain
chemicals, among other things decomposing water
into its elements, hydrogen and oxygen. On May 7,
1800, these investigators arranged the ends of two
brass wires connected with the poles of a voltaic pile,
composed of alternate silver and zinc plates, so that
the current coming from the pile was discharged
through a small quantity of "New River water." "A
fine stream of minute bubbles immediately began
to flow from the point of the lower wire in the tube
which communicated with the silver," wrote Nicholson,
"and the opposite point of the upper wire became
tarnished, first deep orange and then black. . . ." The
product of gas during two hours and a half was two-
thirtieths of a cubic inch. "It was then mixed with
an equal quantity of common air," continues Nicholson,
"and exploded by the application of a lighted
waxen thread."
This demonstration was the beginning of the very
important science of electro-chemistry.
The importance of this discovery was at once recognized
by Sir Humphry Davy, who began experimenting
immediately in this new field. He constructed a
series of batteries in various combinations, with which
he attacked the "fixed alkalies," the composition of
which was then unknown. Very shortly he was able
to decompose potash into bright metallic globules,
resembling quicksilver. This new substance he named
"potassium." Then in rapid succession the elementary
substances sodium, calcium, strontium, and magnesium
were isolated.
It was soon discovered, also, that the new electricity,
like the old, possessed heating power under certain
conditions, even to the fusing of pieces of wire. This
observation was probably first made by Frommsdorff,
but it was elaborated by Davy, who constructed a
battery of two thousand cells with which he produced
a bright light from points of carbon--the prototype of
the modern arc lamp. He made this demonstration
before the members of the Royal Institution in 1810.
But the practical utility of such a light for illuminating
purposes was still a thing of the future. The expense
of constructing and maintaining such an elaborate
battery, and the rapid internal destruction of its plates,
together with the constant polarization, rendered its
use in practical illumination out of the question. It
was not until another method of generating electricity
was discovered that Davy's demonstration could be
turned to practical account.
In Davy's own account of his experiment he says:
"When pieces of charcoal about an inch long and
one-sixth of an inch in diameter were brought near each
other (within the thirtieth or fortieth of an inch), a
bright spark was produced, and more than half the
volume of the charcoal became ignited to whiteness;
and, by withdrawing the points from each other, a constant
discharge took place through the heated air, in a
space equal to at least four inches, producing a most
brilliant ascending arch of light, broad and conical in
form in the middle. When any substance was introduced
into this arch, it instantly became ignited;
platina melted as readily in it as wax in a common candle;
quartz, the sapphire, magnesia, lime, all entered
into fusion; fragments of diamond and points of charcoal
and plumbago seemed to evaporate in it, even
when the connection was made in the receiver of an
air-pump; but there was no evidence of their having
previously undergone fusion. When the communication
between the points positively and negatively electrified
was made in the air rarefied in the receiver of the
air-pump, the distance at which the discharge took
place increased as the exhaustion was made; and when
the atmosphere in the vessel supported only one-
fourth of an inch of mercury in the barometrical gauge,
the sparks passed through a space of nearly half an
inch; and, by withdrawing the points from each other,
the discharge was made through six or seven inches,
producing a most brilliant coruscation of purple light;
the charcoal became intensely ignited, and some platina
wire attached to it fused with brilliant scintillations
and fell in large globules upon the plate of the pump.
All the phenomena of chemical decomposition were
produced with intense rapidity by this combination."[1]
But this experiment demonstrated another thing
besides the possibility of producing electric light and
chemical decomposition, this being the heating power
capable of being produced by the electric current.
Thus Davy's experiment of fusing substances laid the
foundation of the modern electric furnaces, which are
of paramount importance in several great commercial
industries.
While some of the results obtained with Davy's
batteries were practically as satisfactory as could be
obtained with modern cell batteries, the batteries
themselves were anything but satisfactory. They were
expensive, required constant care and attention, and,
what was more important from an experimental standpoint
at least, were not constant in their action except
for a very limited period of time, the current soon
"running down." Numerous experimenters, therefore,
set about devising a satisfactory battery, and
when, in 1836, John Frederick Daniell produced the
cell that bears his name, his invention was epoch-
making in the history of electrical progress. The
Royal Society considered it of sufficient importance
to bestow the Copley medal upon the inventor, whose
device is the direct parent of all modern galvanic cells.
From the time of the advent of the Daniell cell experiments
in electricity were rendered comparatively
easy. In the mean while, however, another great discovery
was made.
ELECTRICITY AND MAGNETISM
For many years there had been a growing suspicion,
amounting in many instances to belief in the close
relationship existing between electricity and magnetism.
Before the winter of 1815, however, it was a belief
that was surmised but not demonstrated. But in that
year it occurred to Jean Christian Oersted, of Denmark,
to pass a current of electricity through a wire
held parallel with, but not quite touching, a suspended
magnetic needle. The needle was instantly deflected
and swung out of its position.
"The first experiments in connection with the subject
which I am undertaking to explain," wrote Oersted,
"were made during the course of lectures which
I held last winter on electricity and magnetism. From
those experiments it appeared that the magnetic needle
could be moved from its position by means of a galvanic
battery--one with a closed galvanic circuit.
Since, however, those experiments were made with an
apparatus of small power, I undertook to repeat and
increase them with a large galvanic battery.
"Let us suppose that the two opposite ends of the
galvanic apparatus are joined by a metal wire. This
I shall always call the conductor for the sake of brevity.
Place a rectilinear piece of this conductor in a horizontal
position over an ordinary magnetic needle so that
it is parallel to it. The magnetic needle will be set in
motion and will deviate towards the west under that
part of the conductor which comes from the negative
pole of the galvanic battery. If the wire is not more
than four-fifths of an inch distant from the middle of
this needle, this deviation will be about forty-five degrees.
At a greater distance the angle of deviation
becomes less. Moreover, the deviation varies according
to the strength of the battery. The conductor can
be moved towards the east or west, so long as it remains
parallel to the needle, without producing any
other result than to make the deviation smaller.
"The conductor can consist of several combined
wires or metal coils. The nature of the metal does not
alter the result except, perhaps, to make it greater or
less. We have used wires of platinum, gold, silver,
brass, and iron, and coils of lead, tin, and quicksilver
with the same result. If the conductor is interrupted
by water, all effect is not cut off, unless the stretch
of water is several inches long.
"The conductor works on the magnetic needle
through glass, metals, wood, water, and resin, through
clay vessels and through stone, for when we placed a
glass plate, a metal plate, or a board between the conductor
and the needle the effect was not cut off; even
the three together seemed hardly to weaken the effect,
and the same was the case with an earthen vessel, even
when it was full of water. Our experiments also demonstrated
that the said effects were not altered when
we used a magnetic needle which was in a brass case
full of water.
"When the conductor is placed in a horizontal plane
under the magnetic needle all the effects we have described
take place in precisely the same way, but in
the opposite direction to what took place when the
conductor was in a horizontal plane above the needle.
"If the conductor is moved in a horizontal plane so
that it gradually makes ever-increasing angles with the
magnetic meridian, the deviation of the magnetic
needle from the magnetic meridian is increased when
the wire is turned towards the place of the needle; it
decreases, on the other hand, when it is turned away
from that place.
"A needle of brass which is hung in the same way as
the magnetic needle is not set in motion by the influence
of the conductor. A needle of glass or rubber likewise
remains static under similar experiments. Hence
the electrical conductor affects only the magnetic
parts of a substance. That the electrical current is
not confined to the conducting wire, but is comparatively
widely diffused in the surrounding space, is
sufficiently demonstrated from the foregoing observations."[2]
The effect of Oersted's demonstration is almost
incomprehensible. By it was shown the close relationship
between magnetism and electricity. It showed
the way to the establishment of the science of electrodynamics;
although it was by the French savant
Andre Marie Ampere (1775-1836) that the science was
actually created, and this within the space of one week
after hearing of Oersted's experiment in deflecting the
needle. Ampere first received the news of Oersted's
experiment on September 11, 1820, and on the 18th
of the same month he announced to the Academy the
fundamental principles of the science of electro-dynamics--
seven days of rapid progress perhaps unequalled
in the history of science.
Ampere's distinguished countryman, Arago, a few
months later, gave the finishing touches to Oersted's
and Ampere's discoveries, by demonstrating conclusively
that electricity not only influenced a magnet,
but actually produced magnetism under proper circumstances
--a complemental fact most essential in
practical mechanics
Some four years after Arago's discovery, Sturgeon
made the first "electro-magnet" by winding a soft
iron core with wire through which a current of electricity
was passed. This study of electro-magnets
was taken up by Professor Joseph Henry, of Albany,
New York, who succeeded in making magnets of enormous
lifting power by winding the iron core with several
coils of wire. One of these magnets, excited by
a single galvanic cell of less than half a square foot
of surface, and containing only half a pint of dilute
acids, sustained a weight of six hundred and fifty
pounds.
Thus by Oersted's great discovery of the intimate
relationship of magnetism and electricity, with further
elaborations and discoveries by Ampere, Volta, and
Henry, and with the invention of Daniell's cell, the
way was laid for putting electricity to practical use.
Soon followed the invention and perfection of the
electro-magnetic telegraph and a host of other but
little less important devices.
FARADAY AND ELECTRO-MAGNETIC INDUCTION
With these great discoveries and inventions at hand,
electricity became no longer a toy or a "plaything for
philosophers," but of enormous and growing importance
commercially. Still, electricity generated by
chemical action, even in a very perfect cell, was both
feeble and expensive, and, withal, only applicable in a
comparatively limited field. Another important scientific
discovery was necessary before such things as
electric traction and electric lighting on a large scale
were to become possible; but that discovery was soon
made by Sir Michael Faraday.
Faraday, the son of a blacksmith and a bookbinder
by trade, had interested Sir Humphry Davy by his
admirable notes on four of Davy's lectures, which he
had been able to attend. Although advised by the
great scientist to "stick to his bookbinding" rather
than enter the field of science, Faraday became, at
twenty-two years of age, Davy's assistant in the Royal
Institution. There, for several years, he devoted all
his spare hours to scientific investigations and experiments,
perfecting himself in scientific technique.
A few years later he became interested, like all the
scientists of the time, in Arago's experiment of rotating
a copper disk underneath a suspended compass-
needle. When this disk was rotated rapidly, the needle
was deflected, or even rotated about its axis, in a manner
quite inexplicable. Faraday at once conceived the
idea that the cause of this rotation was due to electricity,
induced in the revolving disk--not only conceived
it, but put his belief in writing. For several years,
however, he was unable to demonstrate the truth of
his assumption, although he made repeated experiments
to prove it. But in 1831 he began a series of
experiments that established forever the fact of
electro-magnetic induction.
In his famous paper, read before the Royal Society
in 1831, Faraday describes the method by which he first
demonstrated electro-magnetic induction, and then explained
the phenomenon of Arago's revolving disk.
"About twenty-six feet of copper wire, one-twentieth
of an inch in diameter, were wound round a cylinder
of wood as a helix," he said, "the different spires of
which were prevented from touching by a thin interposed
twine. This helix was covered with calico, and
then a second wire applied in the same manner. In this
way twelve helices were "superposed, each containing
an average length of wire of twenty-seven feet, and all
in the same direction. The first, third, fifth, seventh,
ninth, and eleventh of these helices were connected at
their extremities end to end so as to form one helix;
the others were connected in a similar manner; and
thus two principal helices were produced, closely interposed,
having the same direction, not touching anywhere,
and each containing one hundred and fifty-five
feet in length of wire.
One of these helices was connected with a galvanometer,
the other with a voltaic battery of ten pairs
of plates four inches square, with double coppers
and well charged; yet not the slightest sensible
deflection of the galvanometer needle could be observed.
"A similar compound helix, consisting of six lengths
of copper and six of soft iron wire, was constructed.
The resulting iron helix contained two hundred and
eight feet; but whether the current from the trough
was passed through the copper or the iron helix, no
effect upon the other could be perceived at the galvanometer.
"In these and many similar experiments no difference
in action of any kind appeared between iron and
other metals.
"Two hundred and three feet of copper wire in one
length were passed round a large block of wood; other
two hundred and three feet of similar wire were interposed
as a spiral between the turns of the first, and
metallic contact everywhere prevented by twine. One
of these helices was connected with a galvanometer and
the other with a battery of a hundred pairs of plates
four inches square, with double coppers and well
charged. When the contact was made, there was a
sudden and very slight effect at the galvanometer, and
there was also a similar slight effect when the contact
with the battery was broken. But whilst the voltaic
current was continuing to pass through the one helix,
no galvanometrical appearances of any effect like induction
upon the other helix could be perceived, although
the active power of the battery was proved to
be great by its heating the whole of its own helix, and
by the brilliancy of the discharge when made through
charcoal.
"Repetition of the experiments with a battery of
one hundred and twenty pairs of plates produced no
other effects; but it was ascertained, both at this and
at the former time, that the slight deflection of the
needle occurring at the moment of completing the connection
was always in one direction, and that the
equally slight deflection produced when the contact
was broken was in the other direction; and, also, that
these effects occurred when the first helices were used.
"The results which I had by this time obtained with
magnets led me to believe that the battery current
through one wire did, in reality, induce a similar current
through the other wire, but that it continued for
an instant only, and partook more of the nature of the
electrical wave passed through from the shock of a
common Leyden jar than of that from a voltaic battery,
and, therefore, might magnetize a steel needle although
it scarcely affected the galvanometer.
"This expectation was confirmed; for on substituting
a small hollow helix, formed round a glass tube, for the
galvanometer, introducing a steel needle, making contact
as before between the battery and the inducing
wire, and then removing the needle before the battery
contact was broken, it was found magnetized.
"When the battery contact was first made, then an
unmagnetized needle introduced, and lastly the battery
contact broken, the needle was found magnetized to
an equal degree apparently with the first; but the poles
were of the contrary kinds."[3]
To Faraday these experiments explained the phenomenon
of Arago's rotating disk, the disk inducing the
current from the magnet, and, in reacting, deflecting
the needle. To prove this, he constructed a disk that
revolved between the poles of an electro-magnet, connecting
the axis and the edge of the disk with a galvanometer.
". . . A disk of copper, twelve inches in
diameter, fixed upon a brass axis," he says, "was
mounted in frames so as to be revolved either vertically
or horizontally, its edge being at the same time introduced
more or less between the magnetic poles. The
edge of the plate was well amalgamated for the purpose
of obtaining good but movable contact; a part round
the axis was also prepared in a similar manner.
"Conductors or collectors of copper and lead were
constructed so as to come in contact with the edge of the
copper disk, or with other forms of plates hereafter to
be described. These conductors we're about four inches
long, one-third of an inch wide, and one-fifth of an inch
thick; one end of each was slightly grooved, to allow
of more exact adaptation to the somewhat convex edge
of the plates, and then amalgamated. Copper wires,
one-sixteenth of an inch in thickness, attached in the
ordinary manner by convolutions to the other ends of
these conductors, passed away to the galvanometer.
"All these arrangements being made, the copper
disk was adjusted, the small magnetic poles being
about one-half an inch apart, and the edge of the plate
inserted about half their width between them. One
of the galvanometer wires was passed twice or thrice
loosely round the brass axis of the plate, and the other
attached to a conductor, which itself was retained by
the hand in contact with the amalgamated edge of the
disk at the part immediately between the magnetic
poles. Under these circumstances all was quiescent,
and the galvanometer exhibited no effect. But the
instant the plate moved the galvanometer was influenced,
and by revolving the plate quickly the needle
could be deflected ninety degrees or more."[4]
This rotating disk was really a dynamo electric
machine in miniature, the first ever constructed, but
whose direct descendants are the ordinary dynamos.
Modern dynamos range in power from little machines
operating machinery requiring only fractions of a horsepower
to great dynamos operating street-car lines and
lighting cities; but all are built on the same principle
as Faraday's rotating disk. By this discovery the use
of electricity as a practical and economical motive
power became possible.
STORAGE BATTERIES
When the discoveries of Faraday of electro-magnetic
induction had made possible the means of easily generating
electricity, the next natural step was to find a
means of storing it or accumulating it. This, however,
proved no easy matter, and as yet a practical storage
or secondary battery that is neither too cumbersome,
too fragile, nor too weak in its action has not been
invented. If a satisfactory storage battery could be
made, it is obvious that its revolutionary effects could
scarcely be overestimated. In the single field of aeronautics,
it would probably solve the question of aerial
navigation. Little wonder, then, that inventors have
sought so eagerly for the invention of satisfactory storage
batteries. As early as 1803 Ritter had attempted
to make such a secondary battery. In 1843 Grove
also attempted it. But it was not until 1859, when
Gaston Planche produced his invention, that anything
like a reasonably satisfactory storage battery
was made. Planche discovered that sheets of lead
immersed in dilute sulphuric acid were very satisfactory
for the production of polarization effects. He
constructed a battery of sheets of lead immersed in
sulphuric acid, and, after charging these for several
hours from the cells of an ordinary Bunsen battery,
was able to get currents of great strength and considerable
duration. This battery, however, from its construction
of lead, was necessarily heavy and cumbersome.
Faure improved it somewhat by coating the
lead plates with red-lead, thus increasing the capacity
of the cell. Faure's invention gave a fresh impetus
to inventors, and shortly after the market was filled
with storage batteries of various kinds, most of them
modifications of Planche's or Faure's. The ardor of
enthusiastic inventors soon flagged, however, for all
these storage batteries proved of little practical account
in the end, as compared with other known
methods of generating power.
Three methods of generating electricity are in general
use: static or frictional electricity is generated by
"plate" or "static" machines; galvanic, generated by
batteries based on Volta's discovery; and induced, or
faradic, generated either by chemical or mechanical
action. There is still another kind, thermo-electricity,
that may be generated in a most simple manner. In
1821 Seebecle, of Berlin, discovered that when a
circuit was formed of two wires of different metals, if
there be a difference in temperature at the juncture of
these two metals an electrical current will be established.
In this way heat may be transmitted directly
into the energy of the current without the interposition
of the steam-engine. Batteries constructed in
this way are of low resistance, however, although by
arranging several of them in "series," currents of
considerable strength can be generated. As yet, however,
they are of little practical importance.
About the middle of the century Clerk-Maxwell
advanced the idea that light waves were really electro-
magnetic waves. If this were true and light proved
to be simply one form of electrical energy, then the
same would be true of radiant heat. Maxwell advanced
this theory, but failed to substantiate it by
experimental confirmation. But Dr. Heinrich Hertz,
a few years later, by a series of experiments, demonstrated
the correctness of Maxwell's surmises. What
are now called "Hertzian waves" are waves apparently
identical with light waves, but of much lower pitch or
period. In his experiments Hertz showed that, under
proper conditions, electric sparks between polished balls
were attended by ether waves of the same nature as those
of light, but of a pitch of several millions of vibrations
per second. These waves could be dealt with as if they
were light waves--reflected, refracted, and polarized.
These are the waves that are utilized in wireless telegraphy.
ROENTGEN RAYS, OR X-RAYS
In December of 1895 word came out of Germany of
a scientific discovery that startled the world. It came
first as a rumor, little credited; then as a pronounced
report; at last as a demonstration. It told of a new
manifestation of energy, in virtue of which the interior
of opaque objects is made visible to human eyes. One
had only to look into a tube containing a screen of a
certain composition, and directed towards a peculiar
electrical apparatus, to acquire clairvoyant vision more
wonderful than the discredited second-sight of the
medium. Coins within a purse, nails driven into wood,
spectacles within a leather case, became clearly visible
when subjected to the influence of this magic tube; and
when a human hand was held before the tube, its bones
stood revealed in weird simplicity, as if the living, palpitating
flesh about them were but the shadowy substance
of a ghost.
Not only could the human eye see these astounding
revelations, but the impartial evidence of inanimate
chemicals could be brought forward to prove that the
mind harbored no illusion. The photographic film recorded
the things that the eye might see, and ghostly
pictures galore soon gave a quietus to the doubts of the
most sceptical. Within a month of the announcement
of Professor Roentgen's experiments comment
upon the "X-ray" and the "new photography" had
become a part of the current gossip of all Christendom.
It is hardly necessary to say that such a revolutionary
thing as the discovery of a process whereby opaque
objects became transparent, or translucent, was not
achieved at a single bound with no intermediate discoveries.
In 1859 the German physicist Julius Plucker
(1801-1868) noticed that when there was an electrical
discharge through an exhausted tube at a low pressure,
on the surrounding walls of the tube near the negative
pole, or cathode, appeared a greenish phosphorescence.
This discovery was soon being investigated by a number
of other scientists, among others Hittorf, Goldstein,
and Professor (now Sir William) Crookes. The
explanations given of this phenomenon by Professor
Crookes concern us here more particularly, inasmuch
as his views did not accord exactly with those held by
the other two scientists, and as his researches were more
directly concerned in the discovery of the Roentgen
rays. He held that the heat and phosphorescence
produced in a low-pressure tube were caused by streams
of particles, projected from the cathode with great
velocity, striking the sides of the glass tube. The
composition of the glass seemed to enter into this
phosphorescence also, for while lead glass produced
blue phosphorescence, soda glass produced a yellowish
green. The composition of the glass seemed to be
changed by a long-continued pelting of these particles,
the phosphorescence after a time losing its initial
brilliancy, caused by the glass becoming "tired," as
Professor Crookes said. Thus when some opaque substance,
such as iron, is placed between the cathode and
the sides of the glass tube so that it casts a shadow in
a certain spot on the glass for some little time, it is
found on removing the opaque substance or changing
its position that the area of glass at first covered by
the shadow now responded to the rays in a different
manner from the surrounding glass.
The peculiar ray's, now known as the cathode rays,
not only cast a shadow, but are deflected by a magnet,
so that the position of the phosphorescence on the sides
of the tube may be altered by the proximity of a powerful
magnet. From this it would seem that the rays
are composed of particles charged with negative electricity,
and Professor J. J. Thomson has modified the
experiment of Perrin to show that negative electricity
is actually associated with the rays. There is reason
for believing, therefore, that the cathode rays are rapidly
moving charges of negative electricity. It is possible,
also, to determine the velocity at which these particles
are moving by measuring the deflection produced
by the magnetic field.
From the fact that opaque substances cast a shadow
in these rays it was thought at first that all solids were
absolutely opaque to them. Hertz, however, discovered
that a small amount of phosphorescence occurred
on the glass even when such opaque substances as
gold-leaf or aluminium foil were interposed between
the cathode and the sides of the tube. Shortly afterwards
Lenard discovered that the cathode rays can be
made to pass from the inside of a discharge tube to the
outside air. For convenience these rays outside the
tube have since been known as "Lenard rays."
In the closing days of December, 1895, Professor
Wilhelm Konrad Roentgen, of Wurzburg, announced
that he had made the discovery of the remarkable effect
arising from the cathode rays to which reference
was made above. He found that if a plate covered
with a phosphorescent substance is placed near a discharge
tube exhausted so highly that the cathode rays
produced a green phosphorescence, this plate is made
to glow in a peculiar manner. The rays producing
this glow were not the cathode rays, although
apparently arising from them, and are what have since
been called the Roentgen rays, or X-rays.
Roentgen found that a shadow is thrown upon the
screen by substances held between it and the exhausted
tube, the character of the shadow depending upon the
density of the substance. Thus metals are almost
completely opaque to the rays; such substances as
bone much less so, and ordinary flesh hardly so at all.
If a coin were held in the hand that had been interposed
between the tube and the screen the picture
formed showed the coin as a black shadow; and the
bones of the hand, while casting a distinct shadow,
showed distinctly lighter; while the soft tissues produced
scarcely any shadow at all. The value of such
a discovery was obvious from the first; and was still
further enhanced by the discovery made shortly that,
photographic plates are affected by the rays, thus
making it possible to make permanent photographic
records of pictures through what we know as opaque
substances.
What adds materially to the practical value of
Roentgen's discovery is the fact that the apparatus for
producing the X-rays is now so simple and relatively
inexpensive that it is within the reach even of amateur
scientists. It consists essentially of an induction coil
attached either to cells or a street-current plug for generating
the electricity, a focus tube, and a phosphorescence
screen. These focus tubes are made in various
shapes, but perhaps the most popular are in the form
of a glass globe, not unlike an ordinary small-sized
water-bottle, this tube being closed and exhausted,
and having the two poles (anode and cathode) sealed
into the glass walls, but protruding at either end for
attachment to the conducting wires from the induction
coil. This tube may be mounted on a stand at a
height convenient for manipulation. The phosphorescence
screen is usually a plate covered with some
platino-cyanide and mounted in the end of a box of
convenient size, the opposite end of which is so shaped
that it fits the contour of the face, shutting out the
light and allowing the eyes of the observer to focalize
on the screen at the end. For making observations
the operator has simply to turn on the current of electricity
and apply the screen to his eyes, pointing it
towards the glowing tube, when the shadow of any
substance interposed between the tube and the screen
will appear upon the phosphorescence plate.
The wonderful shadow pictures produced on the
phosphorescence screen, or the photographic plate,
would seem to come from some peculiar form of light,
but the exact nature of these rays is still an open question.
Whether the Roentgen rays are really a form of
light--that is, a form of "electro-magnetic disturbance
propagated through ether," is not fully determined.
Numerous experiments have been undertaken to determine
this, but as yet no proof has been found that
the rays are a form of light, although there appears to
be nothing in their properties inconsistent with their
being so. For the moment most investigators are content
to admit that the term X-ray virtually begs the
question as to the intimate nature of the form of energy
involved.
VIII. THE CONSERVATION OF ENERGY
As we have seen, it was in 1831 that Faraday opened
up the field of magneto-electricity. Reversing
the experiments of his predecessors, who had found
that electric currents may generate magnetism, he
showed that magnets have power under certain circumstances
to generate electricity; he proved, indeed,
the interconvertibility of electricity and magnetism.
Then he showed that all bodies are more or less subject
to the influence of magnetism, and that even light
may be affected by magnetism as to its phenomena of
polarization. He satisfied himself completely of the
true identity of all the various forms of electricity, and
of the convertibility of electricity and chemical action.
Thus he linked together light, chemical affinity, magnetism,
and electricity. And, moreover, he knew full
well that no one of these can be produced in indefinite
supply from another. "Nowhere," he says, "is there
a pure creation or production of power without a corresponding
exhaustion of something to supply it."
When Faraday wrote those words in 1840 he was
treading on the very heels of a greater generalization
than any which he actually formulated; nay, he had it
fairly within his reach. He saw a great truth without
fully realizing its import; it was left for others,
approaching the same truth along another path, to point
out its full significance.
The great generalization which Faraday so narrowly
missed is the truth which since then has become familiar
as the doctrine of the conservation of energy--the
law that in transforming energy from one condition to
another we can never secure more than an equivalent
quantity; that, in short, "to create or annihilate energy
is as impossible as to create or annihilate matter;
and that all the phenomena of the material universe
consist in transformations of energy alone." Some philosophers
think this the greatest generalization ever
conceived by the mind of man. Be that as it may, it is
surely one of the great intellectual landmarks of the
nineteenth century. It stands apart, so stupendous
and so far-reaching in its implications that the generation
which first saw the law developed could little appreciate
it; only now, through the vista of half a century,
do we begin to see it in its true proportions.
A vast generalization such as this is never a mushroom
growth, nor does it usually spring full grown from
the mind of any single man. Always a number of
minds are very near a truth before any one mind fully
grasps it. Pre-eminently true is this of the doctrine of
the conservation of energy. Not Faraday alone, but
half a dozen different men had an inkling of it before
it gained full expression; indeed, every man who advocated
the undulatory theory of light and heat was
verging towards the goal. The doctrine of Young and
Fresnel was as a highway leading surely on to the
wide plain of conservation. The phenomena of electro-
magnetism furnished another such highway. But there
was yet another road which led just as surely and
even more readily to the same goal. This was the
road furnished by the phenomena of heat, and the
men who travelled it were destined to outstrip their
fellow-workers; though, as we have seen, wayfarers on
other roads were within hailing distance when the
leaders passed the mark.
In order to do even approximate justice to the men
who entered into the great achievement, we must recall
that just at the close of the eighteenth century Count
Rumford and Humphry Davy independently showed
that labor may be transformed into heat; and correctly
interpreted this fact as meaning the transformation of
molar into molecular motion. We can hardly doubt
that each of these men of genius realized--vaguely, at
any rate--that there must be a close correspondence
between the amount of the molar and the molecular
motions; hence that each of them was in sight of the
law of the mechanical equivalent of heat. But neither
of them quite grasped or explicitly stated what each
must vaguely have seen; and for just a quarter of a
century no one else even came abreast their line of
thought, let alone passing it.
But then, in 1824, a French philosopher, Sadi Carnot,
caught step with the great Englishmen, and took a
long leap ahead by explicitly stating his belief that a
definite quantity of work could be transformed into a
definite quantity of heat, no more, no less. Carnot did
not, indeed, reach the clear view of his predecessors as
to the nature of heat, for he still thought it a form of
"imponderable" fluid; but he reasoned none the less
clearly as to its mutual convertibility with mechanical
work. But important as his conclusions seem now
that we look back upon them with clearer vision, they
made no impression whatever upon his contemporaries.
Carnot's work in this line was an isolated phenomenon
of historical interest, but it did not enter into the
scheme of the completed narrative in any such way as
did the work of Rumford and Davy.
The man who really took up the broken thread where
Rumford and Davy had dropped it, and wove it into
a completed texture, came upon the scene in 1840.
His home was in Manchester, England; his occupation
that of a manufacturer. He was a friend and
pupil of the great Dr. Dalton. His name was James
Prescott Joule. When posterity has done its final
juggling with the names of the nineteenth century,
it is not unlikely that the name of this Manchester
philosopher will be a household word, like the names
of Aristotle, Copernicus, and Newton.
For Joule's work it was, done in the fifth decade of
the century, which demonstrated beyond all cavil that
there is a precise and absolute equivalence between
mechanical work and heat; that whatever the form of
manifestation of molar motion, it can generate a definite
and measurable amount of heat, and no more.
Joule found, for example, that at the sea-level in
Manchester a pound weight falling through seven
hundred and seventy-two feet could generate enough
heat to raise the temperature of a pound of water one
degree Fahrenheit. There was nothing haphazard,
nothing accidental, about this; it bore the stamp of
unalterable law. And Joule himself saw, what others in
time were made to see, that this truth is merely a
particular case within a more general law. If heat cannot
be in any sense created, but only made manifest as a
transformation of another kind of motion, then must
not the same thing be true of all those other forms of
"force"--light, electricity, magnetism--which had
been shown to be so closely associated, so mutually
convertible, with heat? All analogy seemed to urge the
truth of this inference; all experiment tended to confirm
it. The law of the mechanical equivalent of heat
then became the main corner-stone of the greater law
of the conservation of energy.
But while this citation is fresh in mind, we must turn
our attention with all haste to a country across the
Channel--to Denmark, in short--and learn that even
as Joule experimented with the transformation of heat,
a philosopher of Copenhagen, Colding by name, had
hit upon the same idea, and carried it far towards a
demonstration. And then, without pausing, we must
shift yet again, this time to Germany, and consider the
work of three other men, who independently were on
the track of the same truth, and two of whom, it must
be admitted, reached it earlier than either Joule or
Colding, if neither brought it to quite so clear a
demonstration. The names of these three Germans are
Mohr, Mayer, and Helmholtz. Their share in establishing
the great doctrine of conservation must now
claim our attention.
As to Karl Friedrich Mohr, it may be said that his
statement of the doctrine preceded that of any of his
fellows, yet that otherwise it was perhaps least important.
In 1837 this thoughtful German had grasped
the main truth, and given it expression in an article
published in the Zeitschrift fur Physik, etc. But the
article attracted no attention whatever, even from
Mohr's own countrymen. Still, Mohr's title to rank
as one who independently conceived the great truth,
and perhaps conceived it before any other man
in the world saw it as clearly, even though he
did not demonstrate its validity, is not to be disputed.
It was just five years later, in 1842, that Dr. Julius
Robert Mayer, practising physician in the little German
town of Heilbronn, published a paper in Liebig's
Annalen on "The Forces of Inorganic Nature," in
which not merely the mechanical theory of heat, but
the entire doctrine of the conservation of energy, is explicitly
if briefly stated. Two years earlier Dr. Mayer,
while surgeon to a Dutch India vessel cruising in the
tropics, had observed that the venous blood of a
patient seemed redder than venous blood usually is
observed to be in temperate climates. He pondered
over this seemingly insignificant fact, and at last reached
the conclusion that the cause must be the lesser
amount of oxidation required to keep up the body
temperature in the tropics. Led by this reflection to
consider the body as a machine dependent on outside
forces for its capacity to act, he passed on into a novel
realm of thought, which brought him at last to independent
discovery of the mechanical theory of heat,
and to the first full and comprehensive appreciation
of the great law of conservation. Blood-letting, the
modern physician holds, was a practice of very doubtful
benefit, as a rule, to the subject; but once, at least,
it led to marvellous results. No straw is go small that
it may not point the receptive mind of genius to new
and wonderful truths.
MAYER'S PAPER OF 1842
The paper in which Mayer first gave expression to
his revolutionary ideas bore the title of "The Forces
of Inorganic Nature," and was published in 1842. It
is one of the gems of scientific literature, and fortunately
it is not too long to be quoted in its entirety.
Seldom if ever was a great revolutionary doctrine expounded
in briefer compass:
"What are we to understand by 'forces'? and how
are different forces related to each other? The term
force conveys for the most part the idea of something
unknown, unsearchable, and hypothetical; while the
term matter, on the other hand, implies the possession,
by the object in question, of such definite properties as
weight and extension. An attempt, therefore, to render
the idea of force equally exact with that of matter
is one which should be welcomed by all those who desire
to have their views of nature clear and unencumbered
by hypothesis.
"Forces are causes; and accordingly we may make
full application in relation to them of the principle
causa aequat effectum. If the cause c has the effect e,
then c = e; if, in its turn, e is the cause of a second
effect of f, we have e = f, and so on: c = e = f ... = c.
In a series of causes and effects, a term or a part of a
term can never, as is apparent from the nature of an
equation, become equal to nothing. This first property
of all causes we call their indestructibility.
"If the given cause c has produced an effect e equal
to itself, it has in that very act ceased to be--c has become
e. If, after the production of e, c still remained
in the whole or in part, there must be still further
effects corresponding to this remaining cause: the total
effect of c would thus be > e, which would be contrary
to the supposition c = e. Accordingly, since c becomes
e, and e becomes f, etc., we must regard these
various magnitudes as different forms under which
one and the same object makes its appearance. This
capability of assuming various forms is the second
essential property of all causes. Taking both properties
together, we may say, causes an INDESTRUCTIBLE
quantitatively, and quantitatively CONVERTIBLE objects.
"There occur in nature two causes which apparently
never pass one into the other," said Mayer. "The
first class consists of such causes as possess the properties
of weight and impenetrability. These are kinds
of matter. The other class is composed of causes
which are wanting in the properties just mentioned--
namely, forces, called also imponderables, from the
negative property that has been indicated. Forces are
therefore INDESTRUCTIBLE, CONVERTIBLE, IMPONDERABLE OBJECTS.
"As an example of causes and effects, take matter:
explosive gas, H + O, and water, HO, are related
to each other as cause and effect; therefore H + O =
HO. But if H + O becomes HO, heat, cal., makes its
appearance as well as water; this heat must likewise
have a cause, x, and we have therefore H + O + X =
HO + cal. It might be asked, however, whether H + O
is really = HO, and x = cal., and not perhaps H + O =
cal., and x = HO, whence the above equation could
equally be deduced; and so in many other cases. The
phlogistic chemists recognized the equation between
cal. and x, or phlogiston as they called it, and in so doing
made a great step in advance; but they involved
themselves again in a system of mistakes by putting
x in place of O. In this way they obtained H =
HO + x.
"Chemistry teaches us that matter, as a cause, has
matter for its effect; but we may say with equal justification
that to force as a cause corresponds force as
effect. Since c = e, and e = c, it is natural to call one
term of an equation a force, and the other an effect of
force, or phenomenon, and to attach different notions
to the expression force and phenomenon. In brief,
then, if the cause is matter, the effect is matter; if the
cause is a force, the effect is also a force.
"The cause that brings about the raising of a
weight is a force. The effect of the raised weight is,
therefore, also a force; or, expressed in a more general
form, SEPARATION IN SPACE OF PONDERABLE OBJECTS IS A
FORCE; and since this force causes the fall of bodies, we
call it FALLING FORCE. Falling force and fall, or, still more
generally, falling force and motion, are forces related
to each other as cause and effect--forces convertible
into each other--two different forms of one and the
same object. For example, a weight resting on the
ground is not a force: it is neither the cause of motion
nor of the lifting of another weight. It becomes so,
however, in proportion as it is raised above the ground.
The cause--that is, the distance between a weight and
the earth, and the effect, or the quantity of motion
produced, bear to each other, as shown by mechanics,
a constant relation.
'Gravity being regarded as the cause of the falling
of bodies, a gravitating force is spoken of; and thus the
ideas of PROPERTY and of FORCE are confounded with each
other. Precisely that which is the essential attribute
of every force--that is, the UNION of indestructibility
with convertibility--is wanting in every property:
between a property and a force, between gravity and
motion, it is therefore impossible to establish the equation
required for a rightly conceived causal relation.
If gravity be called a force, a cause is supposed which
produces effects without itself diminishing, and incorrect
conceptions of the causal connections of things
are thereby fostered. In order that a body may fall, it
is just as necessary that it be lifted up as that it should
be heavy or possess gravity. The fall of bodies,
therefore, ought not to be ascribed to their gravity
alone. The problem of mechanics is to develop the
equations which subsist between falling force and
motion, motion and falling force, and between different
motions. Here is a case in point: The magnitude
of the falling force v is directly proportional
(the earth's radius being assumed--oo) to the magnitude
of the mass m, and the height d, to which it is
raised--that is, v = md. If the height d = l, to
which the mass m is raised, is transformed into the
final velocity c = l of this mass, we have also v = mc;
but from the known relations existing between d and c,
it results that, for other values of d or of c, the measure
of the force v is mc squared; accordingly v = md = mcsquared. The
law of the conservation of vis viva is thus found to
be based on the general law of the indestructibility of
causes.
"In many cases we see motion cease without having
caused another motion or the lifting of a weight. But
a force once in existence cannot be annihilated--it can
only change its form. And the question therefore
arises, what other forms is force, which we have become
acquainted with as falling force and motion,
capable of assuming? Experience alone can lead us to
a conclusion on this point. That we may experiment
to advantage, we must select implements which, besides
causing a real cessation of motion, are as little as
possible altered by the objects to be examined. For
example, if we rub together two metal plates, we see
motion disappear, and heat, on the other hand, make
its appearance, and there remains to be determined only
whether MOTION is the cause of heat. In order to reach
a decision on this point, we must discuss the question
whether, in the numberless cases in which the expenditure
of motion is accompanied by the appearance of
heat, the motion has not some other effect than the
production of heat, and the heat some other cause
than the motion.
"A serious attempt to ascertain the effects of ceasing
motion has never been made. Without wishing to
exclude a priori the hypothesis which it may be possible
to establish, therefore, we observe only that, as a
rule, this effect cannot be supposed to be an alteration
in the state of aggregation of the moved (that is,
rubbing, etc.) bodies. If we assume that a certain
quantity of motion v is expended in the conversion of
a rubbing substance m into n, we must then have
m + v - n, and n = m + v; and when n is reconverted
into m, v must appear again in some form or other.
By the friction of two metallic plates continued for a
very long time, we can gradually cause the cessation
of an immense quantity of movement; but would it
ever occur to us to look for even the smallest trace of
the force which has disappeared in the metallic dust
that we could collect, and to try to regain it thence?
We repeat, the motion cannot have been annihilated;
and contrary, or positive and negative, motions cannot
be regarded as = o any more than contrary motions
can come out of nothing, or a weight can raise
itself.
"Without the recognition of a causal relation between
motion and heat, it is just as difficult to explain
the production of heat as it is to give any account of
the motion that disappears. The heat cannot be derived
from the diminution of the volume of the rubbing
substances. It is well known that two pieces of ice
may be melted by rubbing them together in vacuo; but
let any one try to convert ice into water by pressure,
however enormous. The author has found that water
undergoes a rise of temperature when shaken violently.
The water so heated (from twelve to thirteen degrees
centigrade) has a greater bulk after being shaken than
it had before. Whence now comes this quantity of
heat, which by repeated shaking may be called into
existence in the same apparatus as often as we please?
The vibratory hypothesis of heat is an approach towards
the doctrine of heat being the effect of motion,
but it does not favor the admission of this causal relation
in its full generality. It rather lays the chief
stress on restless oscillations.
"If it be considered as now established that in many
cases no other effect of motion can be traced except
heat, and that no other cause than motion can be found
for the heat that is produced, we prefer the assumption
that heat proceeds from motion to the assumption
of a cause without effect and of an effect without
a cause. Just as the chemist, instead of allowing
oxygen and hydrogen to disappear without further
investigation, and water to be produced in some
inexplicable manner, establishes a connection between
oxygen and hydrogen on the one hand, and water on
the other.
"We may conceive the natural connection existing
between falling force, motion, and heat as follows:
We know that heat makes its appearance when the
separate particles of a body approach nearer to each
other; condensation produces heat. And what applies
to the smallest particles of matter, and the smallest
intervals between them, must also apply to large
masses and to measurable distances. The falling of a
weight is a diminution of the bulk of the earth, and
must therefore without doubt be related to the quantity
of heat thereby developed; this quantity of heat
must be proportional to the greatness of the weight
and its distance from the ground. From this point of
view we are easily led to the equations between falling
force, motion, and heat that have already been discussed.
"But just as little as the connection between falling
force and motion authorizes the conclusion that the
essence of falling force is motion, can such a conclusion
be adopted in the case of heat. We are, on the contrary,
rather inclined to infer that, before it can
become heat, motion must cease to exist as motion,
whether simple, or vibratory, as in the case of light
and radiant heat, etc.
"If falling force and motion are equivalent to heat,
heat must also naturally be equivalent to motion and
falling force. Just as heat appears as an EFFECT of the
diminution of bulk and of the cessation of motion, so
also does heat disappear as a CAUSE when its effects are
produced in the shape of motion, expansion, or raising
of weight.
"In water-mills the continual diminution in bulk
which the earth undergoes, owing to the fall of the
water, gives rise to motion, which afterwards disappears
again, calling forth unceasingly a great quantity
of heat; and, inversely, the steam-engine serves to
decompose heat again into motion or the raising of
weights. A locomotive with its train may be compared
to a distilling apparatus; the heat applied under
the boiler passes off as motion, and this is deposited
again as heat at the axles of the wheels."
Mayer then closes his paper with the following deduction:
"The solution of the equations subsisting between
falling force and motion requires that the space
fallen through in a given time--e. g., the first second--
should be experimentally determined. In like manner,
the solution of the equations subsisting between falling
force and motion on the one hand and heat on the
other requires an answer to the question, How great
is the quantity of heat which corresponds to a given
quantity of motion or falling force? For instance,
we must ascertain how high a given weight requires to
be raised above the ground in order that its falling
force maybe equivalent to the raising of the temperature
of an equal weight of water from 0 degrees to 1 degrees
centigrade. The attempt to show that such an
equation is the expression of a physical truth may
be regarded as the substance of the foregoing remarks.
"By applying the principles that have been set forth
to the relations subsisting between the temperature
and the volume of gases, we find that the sinking of a
mercury column by which a gas is compressed is equivalent
to the quantity of heat set free by the compression;
and hence it follows, the ratio between the capacity
for heat of air under constant pressure and its capacity
under constant volume being taken as = 1.421,
that the warming of a given weight of water from
0 degrees to 1 degrees centigrade corresponds to the fall of an
equal
weight from the height of about three hundred and
sixty-five metres. If we compare with this result the
working of our best steam-engines, we see how small a
part only of the heat applied under the boiler is really
transformed into motion or the raising of weights; and
this may serve as justification for the attempts at the
profitable production of motion by some other method
than the expenditure of the chemical difference between
carbon and oxygen--more particularly by the
transformation into motion of electricity obtained by
chemical means."[1]
MAYER AND HELMHOLTZ
Here, then, was this obscure German physician, leading
the humdrum life of a village practitioner, yet
seeing such visions as no human being in the world had
ever seen before.
The great principle he had discovered became the
dominating thought of his life, and filled all his leisure
hours. He applied it far and wide, amid all the phenomena
of the inorganic and organic worlds. It taught
him that both vegetables and animals are machines,
bound by the same laws that hold sway over inorganic
matter, transforming energy, but creating nothing.
Then his mind reached out into space and met a universe
made up of questions. Each star that blinked
down at him as he rode in answer to a night-call seemed
an interrogation-point asking, How do I exist? Why
have I not long since burned out if your theory of
conservation be true? No one had hitherto even tried
to answer that question; few had so much as realized
that it demanded an answer. But the Heilbronn physician
understood the question and found an answer.
His meteoric hypothesis, published in 1848, gave for the
first time a tenable explanation of the persistent light
and heat of our sun and the myriad other suns--an
explanation to which we shall recur in another connection.
All this time our isolated philosopher, his brain aflame
with the glow of creative thought, was quite unaware
that any one else in the world was working along the
same lines. And the outside world was equally heedless
of the work of the Heilbronn physician. There
was no friend to inspire enthusiasm and give courage,
no kindred spirit to react on this masterful but lonely
mind. And this is the more remarkable because there
are few other cases where a master-originator in science
has come upon the scene except as the pupil or friend
of some other master-originator. Of the men we have
noticed in the present connection, Young was the friend
and confrere of Davy; Davy, the protege of Rumford;
Faraday, the pupil of Davy; Fresnel, the co-worker
with Arago; Colding, the confrere of Oersted; Joule,
the pupil of Dalton. But Mayer is an isolated
phenomenon--one of the lone mountain-peak intellects of
the century. That estimate may be exaggerated
which has called him the Galileo of the nineteenth
century, but surely no lukewarm praise can do him
justice.
Yet for a long time his work attracted no attention
whatever. In 1847, when another German physician,
Hermann von Helmholtz, one of the most massive and
towering intellects of any age, had been independently
led to comprehension of the doctrine of the conservation
of energy and published his treatise on the subject, he
had hardly heard of his countryman Mayer. When he
did hear of him, however, he hastened to renounce all
claim to the doctrine of conservation, though the
world at large gives him credit of independent even
though subsequent discovery.
JOULE'S PAPER OF 1843
Meantime, in England, Joule was going on from one
experimental demonstration to another, oblivious of his
German competitors and almost as little noticed by his
own countrymen. He read his first paper before the
chemical section of the British Association for the
Advancement of Science in 1843, and no one heeded it in
the least. It is well worth our while, however, to
consider it at length. It bears the title, "On the Calorific
Effects of Magneto-Electricity, and the Mechanical
Value of Heat." The full text, as published in the
Report of the British Association, is as follows:
"Although it has been long known that fine platinum
wire can be ignited by magneto-electricity, it
still remained a matter of doubt whether heat was
evolved by the COILS in which the magneto-electricity
was generated; and it seemed indeed not unreasonable
to suppose that COLD was produced there in order to
make up for the heat evolved by the other part of the
circuit. The author therefore has endeavored to clear
up this uncertainty by experiment. His apparatus
consisted of a small compound electro-magnet, immersed
in water, revolving between the poles of a powerful
stationary magnet. The magneto-electricity developed
in the coils of the revolving electro-magnet
was measured by an accurate galvanometer; and the
temperature of the water was taken before and after
each experiment by a very delicate thermometer.
The influence of the temperature of the surrounding
atmospheric air was guarded against by covering the
revolving tube with flannel, etc., and by the adoption
of a system of interpolation. By an extensive series
of experiments with the above apparatus the author
succeeded in proving that heat is evolved by the coils
of the magneto-electrical machine, as well as by any
other part of the circuit, in proportion to the resistance
to conduction of the wire and the square of the
current; the magneto having, under comparable
circumstances, the same calorific power as the voltaic
electricity.
"Professor Jacobi, of St. Petersburg, bad shown that
the motion of an electro-magnetic machine generates
magneto-electricity in opposition to the voltaic current
of the battery. The author had observed the
same phenomenon on arranging his apparatus as an
electro-magnetic machine; but had found that no additional
heat was evolved on account of the conflict of
forces in the coil of the electro-magnet, and that the
heat evolved by the coil remained, as before, proportional
to the square of the current. Again, by turning
the machine contrary to the direction of the attractive
forces, so as to increase the intensity of the voltaic current
by the assistance of the magneto-electricity, he
found that the evolution of heat was still proportional
to the square of the current. The author discovered,
therefore, that the heat evolved by the voltaic current
is invariably proportional to the square of the current,
however the intensity of the current may be varied
by magnetic induction. But Dr. Faraday has shown
that the chemical effects of the current are simply as
its quantity. Therefore he concluded that in the electro-
magnetic engine a part of the heat due to the
chemical actions of the battery is lost by the circuit,
and converted into mechanical power; and that when
the electro-magnetic engine is turned CONTRARY to the
direction of the attractive forces, a greater quantity
of heat is evolved by the circuit than is due to the
chemical reactions of the battery, the over-plus quantity
being produced by the conversion of the mechanical
force exerted in turning the machine. By a dynamometrical
apparatus attached to his machine, the
author has ascertained that, in all the above cases, a
quantity of heat, capable of increasing the temperature
of a pound of water by one degree of Fahrenheit's
scale, is equal to the mechanical force capable of raising
a weight of about eight hundred and thirty pounds
to the height of one foot."[2]
JOULE OR MAYER?
Two years later Joule wished to read another paper,
but the chairman hinted that time was limited, and
asked him to confine himself to a brief verbal synopsis
of the results of his experiments. Had the chairman
but known it, he was curtailing a paper vastly more
important than all the other papers of the meeting put
together. However, the synopsis was given, and one
man was there to hear it who had the genius to appreciate
its importance. This was William Thomson, the
present Lord Kelvin, now known to all the world as
among the greatest of natural philosophers, but then
only a novitiate in science. He came to Joule's aid,
started rolling the ball of controversy, and subsequently
associated himself with the Manchester experimenter
in pursuing his investigations.
But meantime the acknowledged leaders of British
science viewed the new doctrine askance. Faraday,
Brewster, Herschel--those were the great names in
physics at that day, and no one of them could quite
accept the new views regarding energy. For several
years no older physicist, speaking with recognized
authority, came forward in support of the doctrine of
conservation. This culminating thought of the first
half of the nineteenth century came silently into the
world, unheralded and unopposed. The fifth decade
of the century had seen it elaborated and substantially
demonstrated in at least three different countries, yet
even the leaders of thought did not so much as know
of its existence. In 1853 Whewell, the historian of the
inductive sciences, published a second edition of his
history, and, as Huxley has pointed out, he did not so
much as refer to the revolutionizing thought which even
then was a full decade old.
By this time, however, the battle was brewing. The
rising generation saw the importance of a law which
their elders could not appreciate, and soon it was noised
abroad that there were more than one claimant to the
honor of discovery. Chiefly through the efforts of
Professor Tyndall, the work of Mayer became known
to the British public, and a most regrettable controversy
ensued between the partisans of Mayer and those
of Joule--a bitter controversy, in which Davy's contention
that science knows no country was not always
regarded, and which left its scars upon the hearts and
minds of the great men whose personal interests were
involved.
And so to this day the question who is the chief discoverer
of the law of the conservation of energy is not
susceptible of a categorical answer that would satisfy all
philosophers. It is generally held that the first choice
lies between Joule and Mayer. Professor Tyndall has
expressed the belief that in future each of these men
will be equally remembered in connection with this
work. But history gives us no warrant for such a hope.
Posterity in the long run demands always that its heroes
shall stand alone. Who remembers now that
Robert Hooke contested with Newton the discovery
of the doctrine of universal gravitation? The judgment
of posterity is unjust, but it is inexorable. And
so we can little doubt that a century from now one
name will be mentioned as that of the originator of the
great doctrine of the conservation of energy. The man
whose name is thus remembered will perhaps be spoken
of as the Galileo, the Newton, of the nineteenth century;
but whether the name thus dignified by the final
verdict of history will be that of Colding, Mohr, Mayer,
Helmholtz, or Joule, is not as, yet decided.
LORD KELVIN AND THE DISSIPATION OF ENERGY
The gradual permeation of the field by the great
doctrine of conservation simply repeated the history
of the introduction of every novel and revolutionary
thought. Necessarily the elder generation, to whom
all forms of energy were imponderable fluids, must pass
away before the new conception could claim the field.
Even the word energy, though Young had introduced
it in 1807, did not come into general use till some time
after the middle of the century. To the generality of
philosophers (the word physicist was even less in favor
at this time) the various forms of energy were still
subtile fluids, and never was idea relinquished with
greater unwillingness than this. The experiments of
Young and Fresnel had convinced a large number of
philosophers that light is a vibration and not a substance;
but so great an authority as Biot clung to the
old emission idea to the end of his life, in 1862, and held
a following.
Meantime, however, the company of brilliant young
men who had just served their apprenticeship when the
doctrine of conservation came upon the scene had
grown into authoritative positions, and were battling
actively for the new ideas. Confirmatory evidence
that energy is a molecular motion and not an
"imponderable" form of matter accumulated day by day.
The experiments of two Frenchmen, Hippolyte L.
Fizeau and Leon Foucault, served finally to convince
the last lingering sceptics that light is an undulation;
and by implication brought heat into the same category,
since James David Forbes, the Scotch physicist,
had shown in 1837 that radiant heat conforms to the
same laws of polarization and double refraction that
govern light. But, for that matter, the experiments
that had established the mechanical equivalent of heat
hardly left room for doubt as to the immateriality
of this "imponderable." Doubters had indeed, expressed
scepticism as to the validity of Joule's experiments,
but the further researches, experimental and
mathematical, of such workers as Thomson (Lord Kelvin),
Rankine, and Tyndall in Great Britain, of Helmholtz
and Clausius in Germany, and of Regnault in
France, dealing with various manifestations of heat,
placed the evidence beyond the reach of criticism.
Out of these studies, just at the middle of the century,
to which the experiments of Mayer and Joule had
led, grew the new science of thermo-dynamics. Out of
them also grew in the mind of one of the investigators
a new generalization, only second in importance to the
doctrine of conservation itself. Professor William
Thomson (Lord Kelvin) in his studies in thermodynamics
was early impressed with the fact that
whereas all the molar motion developed through labor
or gravity could be converted into heat, the process is
not fully reversible. Heat can, indeed, be converted
into molar motion or work, but in the process a certain
amount of the heat is radiated into space and lost. The
same thing happens whenever any other form of energy
is converted into molar motion. Indeed, every transmutation
of energy, of whatever character, seems complicated
by a tendency to develop heat, part of which
is lost. This observation led Professor Thomson to his
doctrine of the dissipation of energy, which he formulated
before the Royal Society of Edinburgh in 1852,
and published also in the Philosophical Magazine the
same year, the title borne being, "On a Universal
Tendency in Nature to the Dissipation of Mechanical
Energy."
From the principle here expressed Professor Thomson
drew the startling conclusion that, "since any restoration
of this mechanical energy without more than
an equivalent dissipation is impossible," the universe,
as known to us, must be in the condition of a machine
gradually running down; and in particular that the
world we live on has been within a finite time unfit for
human habitation, and must again become so within a
finite future. This thought seems such a commonplace
to-day that it is difficult to realize how startling
it appeared half a century ago. A generation trained, as
ours has been, in the doctrines of the conservation and
dissipation of energy as the very alphabet of physical
science can but ill appreciate the mental attitude of a
generation which for the most part had not even
thought it problematical whether the sun could continue
to give out heat and light forever. But those
advance thinkers who had grasped the import of the
doctrine of conservation could at once appreciate the
force of Thomson's doctrine of dissipation, and realize
the complementary character of the two conceptions.
Here and there a thinker like Rankine did, indeed,
attempt to fancy conditions under which the energy lost
through dissipation might be restored to availability,
but no such effort has met with success, and in time
Professor Thomson's generalization and his conclusions
as to the consequences of the law involved came to be
universally accepted.
The introduction of the new views regarding the nature
of energy followed, as I have said, the course of
every other growth of new ideas. Young and imaginative
men could accept the new point of view; older philosophers,
their minds channelled by preconceptions,
could not get into the new groove. So strikingly true
is this in the particular case now before us that it is
worth while to note the ages at the time of the revolutionary
experiments of the men whose work has been
mentioned as entering into the scheme of evolution of
the idea that energy is merely a manifestation of matter
in motion. Such a list will tell the story better
than a volume of commentary.
Observe, then, that Davy made his epochal experiment
of melting ice by friction when he was a youth of
twenty. Young was no older when he made his first
communication to the Royal Society, and was in his
twenty-seventh year when he first actively espoused
the undulatory theory. Fresnel was twenty-six when
he made his first important discoveries in the same
field; and Arago, who at once became his champion,
was then but two years his senior, though for a decade
he had been so famous that one involuntarily thinks of
him as belonging to an elder generation.
Forbes was under thirty when he discovered the polarization
of heat, which pointed the way to Mohr, then
thirty-one, to the mechanical equivalent. Joule was
twenty-two in 1840, when his great work was begun;
and Mayer, whose discoveries date from the same year,
was then twenty-six, which was also the age of Helmholtz
when he published his independent discovery of
the same law. William Thomson was a youth just past
his majority when he came to the aid of Joule before
the British Society, and but seven years older when he
formulated his own doctrine of the dissipation of energy.
And Clausius and Rankine, who are usually mentioned
with Thomson as the great developers of thermo-dynamics,
were both far advanced with their novel studies
before they were thirty. With such a list in mind, we
may well agree with the father of inductive science
that "the man who is young in years may be old in
hours."
Yet we must not forget that the shield has a reverse
side. For was not the greatest of observing astronomers,
Herschel, past thirty-five before he ever saw a
telescope, and past fifty before he discovered the heat
rays of the spectrum? And had not Faraday reached
middle life before he turned his attention especially to
electricity? Clearly, then, to make this phrase complete,
Bacon should have added that "the man who is
old in years may be young in imagination." Here,
however, even more appropriate than in the other case
--more's the pity--would have been the application
of his qualifying clause: "but that happeneth rarely."
THE FINAL UNIFICATION
There are only a few great generalizations as yet
thought out in any single field of science. Naturally,
then, after a great generalization has found definitive
expression, there is a period of lull before another forward
move. In the case of the doctrines of energy, the
lull has lasted half a century. Throughout this period,
it is true, a multitude of workers have been delving in
the field, and to the casual observer it might seem as if
their activity had been boundless, while the practical
applications of their ideas--as exemplified, for example,
in the telephone, phonograph, electric light, and so on
--have been little less than revolutionary. Yet the
most competent of living authorities, Lord Kelvin,
could assert in 1895 that in fifty years he had learned
nothing new regarding the nature of energy.
This, however, must not be interpreted as meaning
that the world has stood still during these two generations.
It means rather that the rank and file have been
moving forward along the road the leaders had already
travelled. Only a few men in the world had the range
of thought regarding the new doctrine of energy that
Lord Kelvin had at the middle of the century. The
few leaders then saw clearly enough that if one form of
energy is in reality merely an undulation or vibration
among the particles of "ponderable" matter or of ether,
all other manifestations of energy must be of the same
nature. But the rank and file were not even within
sight of this truth for a long time after they had partly
grasped the meaning of the doctrine of conservation.
When, late in the fifties, that marvellous young Scotchman,
James Clerk-Maxwell, formulating in other words
an idea of Faraday's, expressed his belief that electricity
and magnetism are but manifestations of various
conditions of stress and motion in the ethereal medium
(electricity a displacement of strain, magnetism a whirl
in the ether), the idea met with no immediate popularity.
And even less cordial was the reception given the
same thinker's theory, put forward in 1863, that the
ethereal undulations producing the phenomenon we call
light differ in no respect except in their wave-length
from the pulsations of electro-magnetism.
At about the same time Helmholtz formulated a
somewhat similar electro-magnetic theory of light; but
even the weight of this combined authority could not
give the doctrine vogue until very recently, when the
experiments of Heinrich Hertz, the pupil of Helmholtz,
have shown that a condition of electrical strain may be
developed into a wave system by recurrent interruptions
of the electric state in the generator, and that
such waves travel through the ether with the rapidity
of light. Since then the electro-magnetic theory of
light has been enthusiastically referred to as the greatest
generalization of the century; but the sober thinker
must see that it is really only what Hertz himself
called it--one pier beneath the great arch of conservation.
It is an interesting detail of the architecture,
but the part cannot equal the size of the whole.
More than that, this particular pier is as yet by no
means a very firm one. It has, indeed, been demonstrated
that waves of electro-magnetism pass through
space with the speed of light, but as yet no one has
developed electric waves even remotely approximating
the shortness of the visual rays. The most that can
positively be asserted, therefore, is that all the known
forms of radiant energy-heat, light, electro-magnetism--
travel through space at the same rate of speed,
and consist of traverse vibrations--"lateral quivers,"
as Fresnel said of light--known to differ in length,
and not positively known to differ otherwise. It has,
indeed, been suggested that the newest form of radiant
energy, the famous X-ray of Professor Roentgen's discovery,
is a longitudinal vibration, but this is a mere
surmise. Be that as it may, there is no one now to
question that all forms of radiant energy, whatever
their exact affinities, consist essentially of undulatory
motions of one uniform medium.
A full century of experiment, calculation, and controversy
has thus sufficed to correlate the "imponderable
fluids" of our forebears, and reduce them all to
manifestations of motion among particles of matter.
At first glimpse that seems an enormous change of
view. And yet, when closely considered, that change
in thought is not so radical as the change in phrase
might seem to imply. For the nineteenth-century
physicist, in displacing the "imponderable fluids" of
many kinds--one each for light, heat, electricity,
magnetism--has been obliged to substitute for them one
all-pervading fluid, whose various quivers, waves, ripples,
whirls or strains produce the manifestations
which in popular parlance are termed forms of force.
This all-pervading fluid the physicist terms the ether,
and he thinks of it as having no weight. In effect,
then, the physicist has dispossessed the many imponderables
in favor of a single imponderable--though the
word imponderable has been banished from his vocabulary.
In this view the ether--which, considered as
a recognized scientific verity, is essentially a nineteenth-
century discovery--is about the most interesting thing
in the universe. Something more as to its properties,
real or assumed, we shall have occasion to examine as
we turn to the obverse side of physics, which demands
our attention in the next chapter.
IX. THE ETHER AND PONDERABLE MATTER
"Whatever difficulties we may have in forming
a consistent idea of the constitution of the
ether, there can be no doubt that the interplanetary
and interstellar spaces are not empty, but are occupied
by a material substance or body which is certainly the
largest and probably the most uniform body of which
we have any knowledge."
Such was the verdict pronounced some thirty years
ago by James Clerk-Maxwell, one of the very greatest
of nineteenth-century physicists, regarding the
existence of an all-pervading plenum in the universe,
in which every particle of tangible matter is
immersed. And this verdict may be said to express
the attitude of the entire philosophical world of our
day. Without exception, the authoritative physicists
of our time accept this plenum as a verity, and reason
about it with something of the same confidence they
manifest in speaking of "ponderable" matter or of,
energy. It is true there are those among them who are
disposed to deny that this all-pervading plenum merits
the name of matter. But that it is a something, and
a vastly important something at that, all are agreed.
Without it, they allege, we should know nothing of
light, of radiant heat, of electricity or magnetism;
without it there would probably be no such thing as
gravitation; nay, they even hint that without this
strange something, ether, there would be no such
thing as matter in the universe. If these contentions
of the modern physicist are justified, then this
intangible ether is incomparably the most important
as well as the "largest and most uniform substance or
body" in the universe. Its discovery may well be
looked upon as one of the most important feats of the
nineteenth century.
For a discovery of that century it surely is, in the
sense that all the known evidences of its existence were
gathered in that epoch. True dreamers of all ages
have, for metaphysical reasons, imagined the existence
of intangible fluids in space--they had, indeed, peopled
space several times over with different kinds of
ethers, as Maxwell remarks--but such vague dreamings
no more constituted the discovery of the modern
ether than the dream of some pre-Columbian visionary
that land might lie beyond the unknown waters constituted
the discovery of America. In justice it must
be admitted that Huyghens, the seventeenth-century
originator of the undulatory theory of light, caught a
glimpse of the true ether; but his contemporaries and
some eight generations of his successors were utterly
deaf to his claims; so he bears practically the same
relation to the nineteenth-century discoverers of ether
that the Norseman bears to Columbus.
The true Columbus of the ether was Thomas Young.
His discovery was consummated in the early days of
the nineteenth century, when he brought forward the
first, conclusive proofs of the undulatory theory of light.
To say that light consists of undulations is to postulate
something that undulates; and this something could
not be air, for air exists only in infinitesimal quantity, if
at all, in the interstellar spaces, through which light
freely penetrates. But if not air, what then? Why,
clearly, something more intangible than air; something
supersensible, evading all direct efforts to detect it, yet
existing everywhere in seemingly vacant space, and also
interpenetrating the substance of all transparent liquids
and solids, if not, indeed, of all tangible substances.
This intangible something Young rechristened
the Luminiferous Ether.
In the early days of his discovery Young thought of
the undulations which produce light and radiant heat as
being longitudinal--a forward and backward pulsation,
corresponding to the pulsations of sound--and as such
pulsations can be transmitted by a fluid medium with
the properties of ordinary fluids, he was justified in
thinking of the ether as being like a fluid in its properties,
except for its extreme intangibility. But about
1818 the experiments of Fresnel and Arago with polarization
of light made it seem very doubtful whether the
theory of longitudinal vibrations is sufficient, and it
was suggested by Young, and independently conceived
and demonstrated by Fresnel, that the luminiferous
undulations are not longitudinal, but transverse; and
all the more recent experiments have tended to confirm
this view. But it happens that ordinary fluids--
gases and liquids--cannot transmit lateral vibrations;
only rigid bodies are capable of such a vibration. So it
became necessary to assume that the luminiferous ether
is a body possessing elastic rigidity--a familiar property
of tangible solids, but one quite unknown among fluids.
The idea of transverse vibrations carried with it another
puzzle. Why does not the ether, when set
aquiver with the vibration which gives us the sensation
we call light, have produced in its substance subordinate
quivers, setting out at right angles from the
path of the original quiver? Such perpendicular vibrations
seem not to exist, else we might see around a
corner; how explain their absence? The physicist could
think of but one way: they must assume that the ether is
incompressible. It must fill all space--at any rate, all
space with which human knowledge deals--perfectly full.
These properties of the ether, incompressibility and
elastic rigidity, are quite conceivable by themselves;
but difficulties of thought appear when we reflect upon
another quality which the ether clearly must possess--
namely, frictionlessness. By hypothesis this rigid,
incompressible body pervades all space, imbedding every
particle of tangible matter; yet it seems not to retard
the movements of this matter in the slightest degree.
This is undoubtedly the most difficult to comprehend
of the alleged properties of the ether. The physicist
explains it as due to the perfect elasticity of the ether,
in virtue of which it closes in behind a moving particle
with a push exactly counterbalancing the stress required
to penetrate it in front.
To a person unaccustomed to think of seemingly
solid matter as really composed of particles relatively
wide apart, it is hard to understand the claim that
ether penetrates the substance of solids--of glass, for
example--and, to use Young's expression, which we
have previously quoted, moves among them as freely
as the wind moves through a grove of trees. This
thought, however, presents few difficulties to the mind
accustomed to philosophical speculation. But the
question early arose in the mind of Fresnel whether
the ether is not considerably affected by contact with
the particles of solids. Some of his experiments led
him to believe that a portion of the ether which penetrates
among the molecules of tangible matter is held
captive, so to speak, and made to move along with
these particles. He spoke of such portions of the ether
as "bound" ether, in contradistinction to the great
mass of "free" ether. Half a century after Fresnel's
death, when the ether hypothesis had become an accepted
tenet of science, experiments were undertaken
by Fizeau in France, and by Clerk-Maxwell in England,
to ascertain whether any portion of ether is
really thus bound to particles of matter; but the results
of the experiments were negative, and the question
is still undetermined.
While the undulatory theory of light was still fighting
its way, another kind of evidence favoring the existence
of an ether was put forward by Michael Faraday, who,
in the course of his experiments in electrical and magnetic
induction, was led more and more to perceive definite
lines or channels of force in the medium subject to
electro-magnetic influence. Faraday's mind, like that
of Newton and many other philosophers, rejected the
idea of action at a distance, and he felt convinced that
the phenomena of magnetism and of electric induction
told strongly for the existence of an invisible plenum
everywhere in space, which might very probably be
the same plenum that carries the undulations of light
and radiant heat.
Then, about the middle of the century, came that final
revolution of thought regarding the nature of energy
which we have already outlined in the preceding chapter,
and with that the case for ether was considered to
be fully established. The idea that energy is merely a
"mode of motion" (to adopt Tyndall's familiar phrase),
combined with the universal rejection of the notion of
action at a distance, made the acceptance of a plenum
throughout space a necessity of thought--so, at any
rate, it has seemed to most physicists of recent decades.
The proof that all known forms of radiant energy
move through space at the same rate of speed is
regarded as practically a demonstration that but one
plenum--one ether--is concerned in their transmission.
It has, indeed, been tentatively suggested, by Professor
J. Oliver Lodge, that there may be two ethers,
representing the two opposite kinds of electricity, but
even the author of this hypothesis would hardly claim
for it a high degree of probability.
The most recent speculations regarding the properties
of the ether have departed but little from the early
ideas of Young and Fresnel. It is assumed on all sides
that the ether is a continuous, incompressible body,
possessing rigidity and elasticity. Lord Kelvin has
even calculated the probable density of this ether, and
its coefficient of rigidity. As might be supposed, it is
all but infinitely tenuous as compared with any tangible
solid, and its rigidity is but infinitesimal as compared
with that of steel. In a word, it combines properties
of tangible matter in a way not known in any tangible
substance. Therefore we cannot possibly conceive its
true condition correctly. The nearest approximation,
according to Lord Kelvin, is furnished by a mould of
transparent jelly. It is a crude, inaccurate analogy, of
course, the density and resistance of jelly in particular
being utterly different from those of the ether; but the
quivers that run through the jelly when it is shaken,
and the elastic tension under which it is placed when
its mass is twisted about, furnish some analogy to the
quivers and strains in the ether, which are held to constitute
radiant energy, magnetism, and electricity.
The great physicists of the day being at one regarding
the existence of this all-pervading ether, it would
be a manifest presumption for any one standing without
the pale to challenge so firmly rooted a belief.
And, indeed, in any event, there seems little ground on
which to base such a challenge. Yet it may not be altogether
amiss to reflect that the physicist of to-day is
no more certain of his ether than was his predecessor
of the eighteenth century of the existence of certain
alleged substances which he called phlogiston, caloric,
corpuscles of light, and magnetic and electric fluids.
It would be but the repetition of history should it
chance that before the close of another century the
ether should have taken its place along with these discarded
creations of the scientific imagination of earlier
generations. The philosopher of to-day feels very sure
that an ether exists; but when he says there is "no
doubt" of its existence he speaks incautiously, and
steps beyond the bounds of demonstration. He does
not KNOW that action cannot take place at a distance;
he does not KNOW that empty space itself may not perform
the functions which he ascribes to his space-filling
ether.
Meantime, however, the ether, be it substance or be
it only dream-stuff, is serving an admirable purpose in
furnishing a fulcrum for modern physics. Not alone
to the student of energy has it proved invaluable, but to
the student of matter itself as well. Out of its hypothetical
mistiness has been reared the most tenable
theory of the constitution of ponderable matter which
has yet been suggested--or, at any rate, the one that
will stand as the definitive nineteenth-century guess at
this "riddle of the ages." I mean, of course, the vortex
theory of atoms--that profound and fascinating doctrine
which suggests that matter, in all its multiform
phases, is neither more nor less than ether in motion.
The author of this wonderful conception is Lord Kelvin.
The idea was born in his mind of a happy union
of mathematical calculations with concrete experiments.
The mathematical calculations were largely
the work of Hermann von Helmholtz, who, about the
year 1858, had undertaken to solve some unique problems
in vortex motions. Helmholtz found that a vortex
whirl, once established in a frictionless medium,
must go on, theoretically, unchanged forever. In a
limited medium such a whirl may be V-shaped, with
its ends at the surface of the medium. We may imitate
such a vortex by drawing the bowl of a spoon
quickly through a cup of water. But in a limitless
medium the vortex whirl must always be a closed ring,
which may take the simple form of a hoop or circle, or
which may be indefinitely contorted, looped, or, so to
speak, knotted. Whether simple or contorted, this
endless chain of whirling matter (the particles revolving
about the axis of the loop as the particles of a string
revolve when the string is rolled between the fingers)
must, in a frictionless medium, retain its form and
whirl on with undiminished speed forever.
While these theoretical calculations of Helmholtz
were fresh in his mind, Lord Kelvin (then Sir William
Thomson) was shown by Professor P. G. Tait, of Edinburgh,
an apparatus constructed for the purpose of
creating vortex rings in air. The apparatus, which
any one may duplicate, consisted simply of a box with
a hole bored in one side, and a piece of canvas stretched
across the opposite side in lieu of boards. Fumes of
chloride of ammonia are generated within the box,
merely to render the air visible. By tapping with the
band on the canvas side of the box, vortex rings of the
clouded air are driven out, precisely similar in appearance
to those smoke-rings which some expert tobacco-
smokers can produce by tapping on their cheeks, or to
those larger ones which we sometimes see blown out
from the funnel of a locomotive.
The advantage of Professor Tait's apparatus is its
manageableness and the certainty with which the desired
result can be produced. Before Lord Kelvin's interested
observation it threw out rings of various sizes,
which moved straight across the room at varying rates
of speed, according to the initial impulse, and which behaved
very strangely when coming in contact with one
another. If, for example, a rapidly moving ring overtook
another moving in the same path, the one in advance
seemed to pause, and to spread out its periphery
like an elastic band, while the pursuer seemed to contract,
till it actually slid through the orifice of the other,
after which each ring resumed its original size, and
continued its course as if nothing had happened. When,
on the other hand, two rings moving in slightly different
directions came near each other, they seemed to
have an attraction for each other; yet if they impinged,
they bounded away, quivering like elastic solids. If
an effort were made to grasp or to cut one of these rings,
the subtle thing shrank from the contact, and slipped
away as if it were alive.
And all the while the body which thus conducted
itself consisted simply of a whirl in the air, made visible,
but not otherwise influenced, by smoky fumes.
Presently the friction of the surrounding air wore the
ring away, and it faded into the general atmosphere--
often, however, not until it had persisted for many seconds,
and passed clear across a large room. Clearly, if
there were no friction, the ring's inertia must make it a
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