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° to 18° 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° to 10°. 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° colder than the

air, while a black surface at the same time was only 4° 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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