One of the most important ingredients in the air is the vapor of water. Let us try to see, first of all, how it gets into and out of the air. And in this case, as before, you will find that great questions in science often admit of being simply and readily illustrated by the most familiar things. You may have noticed that on very cold nights the windows of sitting-rooms, or crowded public halls, are apt to be found streaming with water on the inside. Now, in such cases, where does the moisture come from? Certainly not out of the glass. It is derived from the vapor of water present in the air. This word vapor is often used to describe some kind of visible mist or fog. But these visible forms of moisture are not properly vapor in the sense in which the term is used in science. The aqueous vapor of the air is always invisible, even when the air is saturated with it, and only when it passes back into the state of water do you actually see anything. When the invisible vapor dissolved in the air becomes visible, as in mists, clouds, dew, or rain, it is said to be condensed, and this process of liquefaction is called condensation. The quantity of vapor which the air can contain varies according to temperature, warm air being able to hold more than cold air. As the air is cooled, its power of retaining vapor diminishes. When it becomes colder than the temperature at which it is able to keep its supply of vapor dissolved, the excess of vapor is condensed and becomes visible. The temperature at which this takes place is the point of saturation, or dew-point. Perhaps you may ask how it is that the vapor so universally present gets into the atmosphere, and where it comes from. If you pour a little water into a plate, and set it down in the open air, you will note in the course of a day or two, that the water has sensibly diminished. The air has drunk up part of it, and will drink up the whole, if the water is allowed to stand long enough. What takes place from a small quantity of water goes on from every surface of water on the face of the earth, from every brook and river and lake, and from the great sea itself. Water is constantly passing off into vapor, which is received and retained by the air. This process is called evaporation, and the water which passes off into vapor is said to evaporate. Since warm air can hold more vapor than cold air, evaporation must be more vigorous in sunshine than at night, and during summer than during winter. On a dry, bracing day, evaporation goes on rapidly, because the air has not nearly got all the quantity of vapor it can hold in solution. On a damp day, however, when the air contains about as much vapor as it can hold at that particular temperature, evaporation is quite feeble, or ceases altogether. This varying capacity of the air for vapor is the reason why laundresses find so much difference between days, in the ease with which they can have their clothes dried.