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M OTHER AMBROISINE had just been washing some handkerchiefs. After soaping them she rinsed them in clear water, and then wrung them out as dry as she could by energetic twisting and squeezing. That done, were they dry enough, and could they be used just as they were?
This question Uncle Paul put to the children, and they all agreed that the handkerchiefs were still very wet, that in fact they held an amount of water exceeding their own weight. What, then, was to be done in order to make them as dry as linen must be before it can be used?
"You all know," resumed Uncle Paul, "what Mother Ambroisine will do to the handkerchiefs: she will hang them on a line in the sun and in a light current of air, if possible. If conditions are favorable, if the sun is warm and a gentle breeze is blowing, the handkerchiefs will soon dry. Then they will only have to be ironed and put away in one of the bureau drawers.
"If the sun does not shine and it is cold, with no air stirring, the drying will take longer. But finally, sooner or later, the handkerchiefs will all get dry; they will lose the water they soaked up at the beginning.
"Let us take another example. We will set a plate of water in the sun. In summer, when it is warm and clear, the water will all disappear between morning and evening, and the bottom of the plate will be found quite dry. In winter wait a few days, a few weeks perhaps, according to the weather, and the same result will be attained: the plate will, in the end, become quite empty although not a single drop has escaped by leaking.
"But is it necessary, after all, to have recourse to any such experiments as these? Will it not suffice to recall to mind what each one of us has witnessed over and over again? Who does not know the little puddles of water, the stagnant pools, that collect in ruts and hollows whenever there is a fall of rain?
"You go by when the pool is full. Ducks are dabbling in it and frogs croaking; black tadpoles, toads-that-are-to-be, sun themselves on the banks, their backs exposed to the noonday warmth, their bellies in the tepid mud. Strange plants, confervæ, as they are called, display their long tufts of sticky green filaments.
"You go by again a little later and no more ducks are dabbling, no more frogs croaking, no more tadpoles frisking, no more confervæ showing their verdure. All have vanished. The pool is dry. Doubtless the soil has drunk up, little by little, at least a part of the stagnant sheet of water where the toad's black family was disporting itself and where the little ducks came waddling in single file to take their first lessons in swimming; but in many cases this slow infiltration of the water into the ground cannot account for the pool's disappearance.
"It may be that the bottom of the natural basin in which the rain-water has collected is formed of compact earth—of clay, for example, which is absolutely impervious to water—or it may be the bottom is of solid rock which, by its very nature, allows not the slightest infiltration.
"How, then, has the pool disappeared? What has become of the water it contained, if the earth has not drunk it up? There were, perhaps, thousands of liters, and now it is all gone. A thirsty chaffinch would not find enough water there to wet its throat. What, too, became of the plateful of water set in the sun, and the moisture in the linen washed by Mother Ambroisine?
"To find the answer to this question, which will lead us farther than you think, it is enough to recall what happens when a pot full of water is put on the fire. The liquid first gets warm and then begins to boil, while from the pot there burst turbulent jets of what looks like white smoke, hot and damp, and known to every one as steam.
"Now, this white smoke, this steam, is water, nothing but water; yet water under another form, water that, instead of running or dripping, expands in the air, floating there as light and thin as the air itself, and becomes dissipated until not a particle of it is visible.
"Watch a puff of steam coming out of the pot. You can see plainly enough the jet of white vapor at the mouth of the vessel, but a little higher up you see nothing whatever, the white puff having become dissipated in the air and being henceforth lost to sight. We no longer see the steam; nevertheless it still exists, whether it has been dispersed through the room, whence it will escape by the doors and windows, or has been carried up the chimney by the current of air ascending from the fireplace.
"Thus the pot loses its contents through its mouth; little by little it empties itself from above; it yields its water to the atmosphere under the form of vapor. The hotter the fire the more rapid the loss. Always losing in this way and receiving nothing, the pot must sooner or later become dry. If the cook does not watch it and, before it is too late, replace the water that has boiled away, the vegetables she has put in it to cook will burn.
"What are we to conclude from this instance of the pot of water on the fire? This: the heat reduces water to vapor or, in other words, to something as thin and invisible as air itself. I insist on this word invisible because—note this well—the white smoke we distinctly see rising from the pot is not yet real vapor.
"Let us call it, if you like, imperfect vapor, or visible vapor, or mist. But when the white smoke has been dissipated in the air and become so thin and limpid that the eye can no longer detect it, then it is real vapor.
"What the intense heat of the fire accomplishes in a short time the sun also effects, but more slowly. It is the sun's heat, then, that dries up the pool by changing its water into vapor; it is the sun's heat that dries the plate by reducing its contents to vapor; it is the sun's heat that dries the linen hung on the line by converting the moisture it holds into vapor.
"Vapor from the pool, from the plate, from the linen, from the boiling pot, all goes into the air and floats there invisible, driven hither and thither by the least puff of wind. The greater the heat, the more rapid the transformation of water into vapor, and also the greater the air's capacity for receiving this charge of invisible moisture.
"That is why the duck-pond dries up sooner in summer than in winter, and why the linen that dries so quickly on a hot day is very slow in drying on a cold and dull one.
"But, whatever the temperature, air cannot receive an unlimited quantity of vapor. When it has a certain amount it becomes too damp to absorb a fresh supply of moisture.
"A perfectly dry sponge readily drinks up a certain quantity of water; already wet, it can take up only a small quantity; and if entirely saturated, it will take up none at all. A pile of dry sand, with its base resting in water, gradually becomes damp to the very top, and when it is thus wet through it cannot absorb any more water.
"Air behaves in the same manner: in a dry state it readily absorbs vapor; saturated to a certain point, it will receive no more. It is a soaked sponge in its powerlessness to drink up more water. So you can easily understand why air in motion, that is to say wind, accelerates the drying of linens and the disappearance of the water in a pond.
"The more humid the air, the less rapidly will it receive vapor, the formation of which is thus arrested; but if the air in conduct with the pond, or with wet linen, or with any sheet of water, be constantly renewed by a breeze, the damp air is succeeded by dry, which in its turn becomes charged with vapor and gives place to other air, and this also carries on the drying process. Thus the transformation of water into vapor proceeds uninterruptedly.
"Let us sum up what we have just learned. Heat changes water into vapor, that is to say into something light and thin, which floats in the air and becomes dissipated until it is as invisible as the air itself. This change is called evaporation. Water evaporates at any temperature, but the more rapidly the greater the heat."