U 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 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,
"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
"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
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."