286. =Evaporation.=--There are two ways in which the change of a liquid into a vapor occurs. One is a rapid change when heat is so applied as to raise the liquid to its boiling point. This is commonly termed vaporization. The other mode is the ordinary gradual evaporation which goes on from the _surface_ of the liquid. This process is going on continuously, not requiring any particular degree of heat, but occurring under all degrees of the temperature of a liquid. Its rapidity, however, is in proportion to the degree of heat, as may be seen by the rise of vapor from water that is being heated, long before it begins to boil. The same thing can also be seen in a bright summer's morning, when the heat of the sun causes the moisture gathered from rain or dew to rise so abundantly from fences, and boards, and roofs as to be visible like smoke. 287. =Solution of Water in Air.=--Evaporation is constantly going on from every wet surface, except when the air is so loaded with moisture that it can take up no more. The vapor is not ordinarily visible, the particles of water passing quietly upward among those of the air, being dissolved in the air just as some solids are dissolved in water. It becomes visible only when so much of it rises that the solution of the water in the air is not readily effected. The readiness with which the solution takes place depends much upon the temperature of the atmosphere. Some very common phenomena illustrate this. In a very cold day the breath of animals, as it comes out of the mouth, seems to be loaded with moisture. Why? It is not because there is more moisture in it than in warm weather, but because cold air can not hold in solution so much water as warm air can. The same explanation applies to the smoking of wet fences and roofs in the sun of a summer's morning. The moisture is heated by the sun, but the air, not having become very warm as yet, can not readily dissolve all the moisture that rises. The phenomenon is not apt to occur when the hot sun shines after a shower at mid-day or in the afternoon, because then the air is warm enough to take up all the moisture that is sent up into it. How water, being heavier than air, rises in the atmosphere is a mystery. It has been supposed by some that it was owing to a kind of affinity existing between water and air. But in opposition to this is the fact that evaporation takes place more rapidly under the exhausted receiver of an air-pump, where there is almost no air, than it does where it is freely exposed to the atmosphere.