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.” ------------------------------------------------------------------------ CHAPTER XLIV RAIN “THE explanations of this morning account for the formation of clouds. A continual evaporation takes place on the surface of the damp earth as well as on the surface of the different sheets of water, lakes, ponds, marshes, streams, and above all the sea. The vapors formed rise into the air and remain invisible as long as the heat is sufficient. But since heat diminishes as the height increases, there comes a time when the vapors can no longer be kept in complete solution, and they condense into a mass of visible vapor, into a fog or cloud.