THE EXPERIMENT WITH THE BOTTLE OF COLD WATER
UNCLE PAUL had rightly said, the evening before, that clouds are nothing
but fog floating high in the air instead of spreading over the earth;
but he had not said what fogs are composed of and how formed. So the
next day he continued his talk on clouds.
“When Mother Ambroisine hangs the clothes she has just washed on the
line, what does she do it for? To dry the linen, to free it from the
water with which it is saturated. Well, what becomes of this water, if
you please?”
“It disappears, I know,” answered Jules, “but I should find it very hard
to tell what becomes of it.”
“This water is dissipated in the air, where it dissolves and becomes as
invisible as the air itself. When you wet a heap of dry sand, the water
permeates it throughout and disappears. It is true that the sand then
takes a different appearance: it was dry before, it is wet afterward.
The sand drinks the water that comes into contact with it. Air does the
same: it drinks the moisture from the linen and becomes damp itself; and
it drinks it so completely that all—air and water—remain as invisible as
if the air held no foreign substance. Vapor is the name given to water
thus made invisible, or in some sort aërial, that is to say resembling
the air; and the reduction of water to this new state is called
evaporation. The moisture of the linen we wish to dry evaporates; the
water is dissipated in the air and thus becomes invisible vapor, which
spreads in every direction at the will of the wind. The warmer it is,
the quicker and more abundant the evaporation. Have you not noticed that
a wet handkerchief dries very quickly in a hot sun, and loses its
moisture only very slowly if the weather is cloudy and cold?”
“Mother Ambroisine is always very glad when she has a fine day for her
washing,” Claire remarked.
“Remember, too, what happens after watering the garden. When, at close
of a very warm day, we have to give a drink to those poor plants dying
of thirst, something like this happens: The pump runs at its utmost
capacity; you all make haste with your watering-pots; one goes here,
another there, carrying water to the suffering plants, seed-plots, and
potted flowers. Soon the garden has drunk copiously. How fresh it is
then, how the plants, wilted by the heat, regain vigor and straighten up
again, as happy as ever! You could almost think you heard them
whispering to one another and telling how glad they were to be watered.
If it could only stay that way! But, bah! the next day the earth is dry
once more and all has to be done over again. What has become of the last
evening’s water? It has evaporated, dissolved into the air; and now it
is perhaps traveling far away, at a great height, until, turned into a
scrap of cloud, it falls again in rain. When Jules tires himself working
the pump to water the flowers, has he ever thought that the water drawn
from the well and spread over the ground sooner or later is dissipated
in the immensities of the air to play its modest part in the formation
of clouds?”
“In watering my garden,” answered Jules, “I did not think I was watering
the air more than anything else. But I see now: air is the great
drinker. Of the contents of a watering-pot the plants take perhaps a
handful; the air drinks up the rest. And that is why we have to do it
all over again every day.”
“And if you exposed a plateful of water to the sun what would finally
become of it?”
“I will answer that,” Emile hastened to reply. “Little by little, the
water would turn into invisible vapor and there would be nothing but the
plate left.”
“What takes place at the expense of a plate of water, and of the
moisture of the soil or wet linen, takes place also, on a vast scale,
over the entire surface of the earth. The air is in contact with damp
soil, with innumerable sheets of water, lakes, marshes, streams, rivers,
brooks, above all with the sea, the immense sea, which presents thrice
as much surface as the dry land. The great drinker, as Jules calls it,
the air, must therefore drink to satiety and everywhere and always
contain moisture, sometimes more, sometimes less, according to the heat.
“The air that is around us now, that invisible air in which the eye
distinguishes nothing, nevertheless contains water that can be made
visible. The means is very simple; all that is necessary is to cool the
air a little. When you squeeze a wet sponge with the hand, you make
water ooze out of it. Cold acts on moist air very much as the pressure
of the hand on the sponge: it causes the moisture to distil in the form
of minute drops. If Claire will go to the pump and fill a bottle with
very cold water, I will show you this curious experiment.”
Claire went to the kitchen and came back with a bottle full of the
coldest water possible. Her uncle took the bottle, wiped it well with
his handkerchief so that no trace of moisture should remain on the
outside, and put it on an equally well-wiped plate.
Now the bottle, at first perfectly clear, becomes covered with a kind of
fog which tarnishes its transparency: then little drops appear, run down
its sides, and fall into the plate. At the end of a quarter of an hour
there was enough water accumulated in the plate to fill a thimble.
“The drops of water now running down the outside of the bottle,” Uncle
Paul explained, “do not come, it is very clear, from the inside, for
glass cannot be pierced by water. They come from the surrounding air,
which cools off on touching the bottle and lets its moisture distil. If
the bottle were colder, if full of ice, the deposit of liquid drops
would be more abundant.”
“The bottle reminds me of something of the same kind,” said Claire.
“When you fill a perfectly clean glass with very cold water, the outside
of the glass immediately tarnishes and looks as if badly washed.”
“That again is the surrounding air depositing its moisture on the cold
side of the glass.”
“Is that invisible moisture contained in the air abundant?” asked Jules.
“The invisible vapor of the air is always a thing so subtle, so
disseminated, that it would take enormous volumes to make a small
quantity of water. During the heat of summer, when the air holds the
most vapor, it takes 60,000 liters of moist air to furnish one liter of
water.”
“That is very little,” was Jules’s comment.
“It is a great deal if one thinks of the immense volume of the
atmosphere,” replied his uncle, and then added:
“The experiment of the bottle teaches us two things: first, there is
always invisible vapor in the air; in the second place, this vapor
becomes visible and changes into fog, then into drops of water, by
cooling. This return of invisible vapor to visible vapor or fog, then to
a state of water, is called condensation. Heat reduces water to
invisible vapor, and cold condenses this vapor, that is to say brings it
back to a liquid state or at least to the state of visible vapor or fog.
We will have the rest this evening.”
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