Scientific American Supplement, No. 530, February 27, 1886
by
Various
Part 3 out of 3
In order that the effect may be good, it is necessary that the objects be
not uniform. Their surface must be naturally carved and hollowed, and the
projecting parts must detach themselves well from each other. The number
of species now used is relatively large, but a selection from these will
inevitably be made. Some patterns will be better liked than others, and
ladies who are to wear these new trimmings this winter will be able to
make their choice of them at the fashion stores. When such articles as
these make their appearance, they often spread with surprising rapidity.
It is now but a few days since the great dressmaker Worth adopted them,
and the linen trade already has them in stock. We recently saw at
Suzange's some linen aprons and collars ornamented with small groups of
fruits and seeds prepared by the Collin process, and which produced a
most pleasing effect. The idea has even occurred to apply these trimmings
to furniture and upholstery.
In the manufacture of these articles the cones of several species of
_Casuarina_, the tags of alder, as well as the naturally carved fruits of
certain _Eloeocarpi_ of India and Australia, were first used; then came
the fruits of the umbelliferous plant, _Oenanthe_, the spiral pods of
_Medicago_, the fruit of the water-caltrops, _Melia_ and _Zizyphus_, the
cups of the acorn, the involucres of the beech, the seeds of _Coix
lacryma_, etc.
The naturalist ought to be glad to see objects that form the base of his
studies taking a direction favorable to the industry of his country.
On another hand, these products themselves cannot fail to arouse the
curiosity of ladies who have the instinct of observation. And, who knows?
Perhaps a frock or mantle trimmed with these vegetable ornaments may
prove a more certain propaganda in favor of botany than the most classic
lessons on this gentle, science!--_La Nature_.
* * * * *
DEW.
[Footnote: Abstract of paper read before the Royal Society of
Edinburgh on Dec. 21, 1885, by Mr. Aitken, communicated by permission of
the Council of the Society.--_Nature_.]
The first point referred to in this paper is the source of the vapor that
condenses to form dew. A short historical sketch is given of the
successive theories from time to time advanced on this point, showing
how in early times dew was supposed to descend from the heavens, and then
afterward it was suggested that it rose from the earth, while Dr. Wells,
who has justly been considered the great master of this subject, thought
it came neither from above nor from below, but was condensed out of the
air near the surface of the earth. He combated Gersten's idea that it
rose from the earth, and showed that all the phenomena observed by
Gersten and others which were advanced to support this theory could be
equally well explained according to the theory that it was simply formed
from the vapor present at the time in the air, and which had risen from
the ground during the day, and concluded that if any did rise from the
ground during night, the quantity must be small, but, with great caution,
he adds that "he was not acquainted with any means of determining the
proportion of this part to the whole."
A few observations of the temperature of the ground near the surface, and
of the air over it, first raised doubts as to the correctness of the now
generally received opinion that dew is formed of vapor existing at the
time in the air. These observations, made at night, showed the ground at
a short distance below the surface to be always hotter than the air over
it, and it was thought that so long as this excess is sufficient to keep
the temperature of the surface of the ground above the dew point of the
air, it will, if moist, give off vapor, and it will be this rising vapor
that will condense on the grass and form dew, and not the vapor that was
previously present in the air.
The first question to be determined was whether vapor does, or does not,
rise from the ground on dewy nights. One method tried of testing this
point was by placing over the grass, in an inverted position, shallow
trays made of thin metal and painted. These trays were put over the
ground to be tested after sunset and examined at night, and also next
morning. It was expected that, if vapor was rising from the ground during
dewy nights, it would be trapped inside the trays. The result in all the
experiments was that the inside was dewed every night, and the grass
inside was wetter than that outside. On some nights there was no dew
outside the trays, and on all nights the inside deposit was heavier than
the outside one.
An analysis of the action of these trays is given, and it is concluded
that they act very much the same as if the air was quite still. Under
these conditions vapor will rise from the ground so long as the
vapor-tension on the surface of the ground is higher than that at the top
of the grass, and much of this rising vapor is, under ordinary
conditions, carried away by the passing air, and mixed with a large
amount of drier air, whereas the vapor rising under the trays is not so
diluted; and hence, though only cooled to the same amount as the air
outside, it yields a heavier deposit of dew.
Another method of testing this point was employed, which consisted in
weighing a small area of the exposed surface of the ground, as it was
evident that if the soil gave off vapor during a dewy night, it must lose
weight. A small turf about 6 inches (152 mm.) square was cut out of the
lawn, and placed in a small shallow pan of about the same size. The pan
with its turf, after being carefully weighed, was put out on the lawn in
the place where the turf had been cut. It was exposed for some hours
while dew was forming, and on these occasions it was always found to lose
weight. It was thus evident that vapor was rising from the ground while
dew was forming, and therefore the dew found on the grass was formed of
part of the rising vapor, trapped or held back by coming into contact
with the cold blades of grass.
The difference between these experiments, in which the exposed bodies
_lose_ weight, and the well-known ones in which bodies are exposed to
radiation, and the amount of dew formed is estimated by the _increase_ in
their weight, is pointed out. In the former case, the bodies are in good
heat-communication with the ground, whereas in the latter little or no
heat is received by conduction from the earth.
Another method employed for determining whether the conditions found in
nature were favorable for dew rising from the ground on dewy nights was
by observations of the temperatures indicated by two thermometers, one
placed on the surface of the grass and the other under the surface, among
the stems, but on the top of the soil. The difference in the readings of
these two thermometers on dewy nights was found to be very considerable.
From 10 deg. to 18 deg. F. was frequently observed. A minimum thermometer placed
on, and another under, the grass showed that during the whole night a
considerable difference was always maintained. As a result of this
difference of temperature, it is evident that vapor will rise from the
hotter soil underneath into the colder air above, and some of it will be
trapped by coming into contact with the cold grass.
While the experiments were being conducted on grass land, parallel
observations were made on bare soil. Over soil the inverted traps
collected more dew inside them than those over grass. A small area of
soil was spread over a shallow pan, and after being weighed was exposed
at the place where the soil had been taken out, to see if bare soil as
well as grass lost weight during dewy nights. The result was that on all
nights on which the tests were made the soil lost weight, and lost very
nearly the same amount as the grass-land.
Another method employed of testing whether vapor is rising from bare
soil, or is being condensed upon it, consisted in placing on the soil,
and in good contact with it, small pieces of black mirror, or any
substance having a surface that shows dewing easily. In this way a small
area of the surface of the earth is converted into a hygroscope, and
these test surfaces tell us whether the ground is cooled to the dew-point
or not. So long as they remain clear and undewed, the surface of the soil
is hotter than the dew-point, and vapor is being given off, while if they
get dewed, the soil will also be condensing vapor. On all nights
observed, these test-surfaces kept clear, and showed the soil to be
always giving off vapor.
All these different methods of testing point to the conclusion that
during dewy nights, in this climate, vapor is constantly being given off
from grass land, and almost always from bare soil; that the tide of vapor
almost always sets outward from the earth and but rarely ebbs, save after
being condensed to cloud and rain, or on those rarer occasions on which,
after the earth has got greatly cooled, a warm moist air blows over it.
The results of the experiments are given, showing, from weighings, the
amount of vapor lost by the soil at night, and also the heat lost by the
surface soil.
It seems probable that when the radiation is strong, that soil,
especially if it is loose and not in good heat-communication with the
ground, will get cooled below the dew-point, and have vapor condensed
upon it. On some occasions the soil certainly got wetter on the surface,
but the question still remains, Whence the vapor? Came it from the air,
or from the soil underneath? The latter seems the more probable source;
the vapor rising from the hot soil underneath will be trapped by the cold
surface-soil, in the same way as it is trapped by grass over grass-land.
During frost, opportunities are afforded of studying this point in a
satisfactory manner, as the trapped vapor keeps its place where it is
condensed. On these occasions the under sides of the clods, at the
surface of the soil, are found to be thickly covered with hoar-frost,
while there is little on their upper or exposed surfaces, showing that
the vapor condensed on the surface-soil has come from below.
The next division of the subject is on dew on roads. It is generally said
that dew forms copiously on grass, while none is deposited on roads,
because grass is a good radiator and cools quicker, and cools more, than
the surface of a road. It is shown that the above statement is wrong, and
that dew really does form abundantly on roads, and that the reason it has
not been observed is that it has not been sought for at the correct
place. We are not entitled to expect to find dew on the surface of roads
as on the surface of grass. because stones are good conductors of heat,
and, the vapor-tension being higher underneath than above the stones, the
result is, the rising vapor gets condensed on the under sides of the
stones. If a road is examined on a dewy night, and the gravel turned up,
the under sides of the stones are found to be dripping wet.
Another reason why no dew forms on the surface of roads is that the
stones, being fair conductors, and in heat communication with the ground,
the temperature of the surface of the road is, from observations taken on
several occasions, higher than that of the surface of the grass
alongside. The air in contact with the stones is, therefore, not cooled
so much as that in contact with the grass.
For studying the formation of dew on roads, slates were found to be
useful. One slate was placed over a gravelly part of the road, and
another over a hard dry part. Examined on dewy nights, the under sides of
these slates were always found to be dripping wet, while their upper
surfaces, and the ground all round, were quite dry.
The importance of the heat communicated from the ground is illustrated by
a simple experiment with two slates or two iron weights, one of them
being placed on the ground, either on grass or on bare soil, and the
other elevated a few inches above the surface. The one resting on the
ground, and in heat-communication with it, is found always to keep dry on
dewy nights, whereas the elevated one gets dewed all over.
The effect of wind in preventing the formation of dew is referred to. It
is shown that, in addition to the other ways already known, wind hinders
the formation of dew by preventing an accumulation of moist air near the
surface of the ground.
An examination of the different forms of vegetation was made on dewy
nights. It was soon evident that something else than radiation and
condensation was at work to produce the varied appearances then seen on
plants. Some kinds of plants were found to be wet, while others of a
different kind, and growing close to them, were dry, and even on the same
plant some branches were wet, while others were dry. The examination of
the leaf of a broccoli plant showed better than any other that the
wetting was not what we might expect if it were dew. The surface of the
leaf was not wet all over, and the amount of deposit on any part had no
relation to its exposure to radiation or access to moist air; but the
moisture was collected in little drops, placed at short distances apart,
along the very edge of the leaf. Closer examination showed that the
position of these drops had a close relation to the structure of the
leaf; they were all placed at the points where the veins in the leaf came
to the outer edge, at once suggesting that these veins were the channels
through which the liquid had been expelled. An examination of grass
revealed a similar condition of matters; the moisture was not equally
distributed over the blade, but was in drops attached to the tips of some
of the blades. These drops, seen on vegetation on dewy nights, are
therefore not dew at all, but are an effect of the vitality of the plant.
It is pointed out that the excretion of drops of liquid by plants is no
new discovery, as it has been long well known, and the experiments of Dr.
Moll on this subject are referred to; but what seems strange is that the
relation of it to dew does not seem to have been recognized.
Some experiments were made on this subject in its relation to dew. Leaves
of plants that had been seen to be wet on dewy nights were experimented
on. They were connected by means of an India-rubber tube with a head of
water of about one meter, and the leaf surrounded with saturated air. All
were found to exude a watery liquid after being subjected to pressure for
some hours, and a broccoli leaf got studded all along its edge with
drops, and presented exactly the same appearance it did on dewy nights. A
stem of grass was also found to exude at the tips of one or two blades
when pressure was applied.
The question as to whether these drops are really exuded by the plant, or
are produced in some other way, is considered. The tip of a blade of
grass was put under conditions in which it could not extract moisture
from the surrounding air, and, as the drop grew as rapidly under these
conditions as did those on the unprotected blades, it is concluded that
these drops are really exuded by the plant. Grass was found to get
"dewed" in air not quite saturated.
On many nights no true dew is formed, and nothing but these exuded drops
appear on the grass; and on all nights when vegetation is active, these
drops appear before the true dew; and if the radiation is strong enough
and the supply of vapor sufficient, true dew makes its appearance, and
now the plants get equally wet all over, in the same manner as dead
matter. The difference between true dew on grass and these exuded drops
can be detected at a glance. The drops are always exuded at a point near
the tip of the blade, and form a drop of some size, while true dew is
distributed all over the blade. The exuded liquid forms a large
diamond-like drop, while the dew coats the blade with a pearly luster.
Toward the end of the paper the radiating powers of different surfaces at
night is considered, and after a reference to some early experiments on
this subject, the paper proceeds to describe some experiments made with
the radiation thermometer described by the author in a previous paper.
When working with this instrument, it is placed in a situation having a
clear view of the sky all round, and is fixed at the same height as the
ordinary thermometer screen, which is worked along with it, the
difference between the thermometer in the screen and the radiation
thermometer being observed. This difference in clear nights amounts to
from 7 deg. to 10 deg.. By means of the radiation thermometer the radiating
powers of different surfaces were observed. Black and white cloths were
found to radiate equally well; soil and grass were also almost exactly
equal to each other. Lampblack was equal to whitening. Sulphur was about
two-thirds of black paint, and polished tin about one-seventh of black
paint. Snow in the shade on a bright day was at midday 7 deg. colder than the
air, while a black surface at the same time was only 4 deg. colder. This
difference diminished as the sun got lower, and at night both radiated
almost equally well. In the concluding pages of the paper some less
important subjects are considered.
* * * * *
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