THE EXPERIMENT WITH THE BOTTLE OF COLD WATER
NCLE 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 of 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
invisi-  ble, 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
 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
"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
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?"
"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."