EVAPORATION

the proce of changing a liquid into vapour, which goes on at the surface of the liquid. hen evaporation takes place into a closed space above the liquid, a state is reached in which the space is saturated with vapour,and no further change is apparent. Certain conditions influence the rate at which evaporation takes place in the open, euch as the extent of the exposed surface, the dryne , pre ure, and temperature of the air, and the Presence cr absence of wind. The farmer looks for a drying wind to dry his corn-sheaves before they are a) EVENING-PRIMROSE carted from the feld. Evaporation is always accompanied by an absorption of heat by the vapour, namely the latent heat nece ary for the change of state from liquid to vapour. This heat is abstracted from ths liquid, which, as a consequence, falis in temperature, and neighbouring bodies may also be cooled. A bather emerging from the water experiences this cocling as the water evaporates from his skin. In hot countries, water is kept cool by being placed in porous earthenware jars: the evaporation which takes place at the outer surface prevents the water in the jar from becoming too warm. The continual evaporation from land and water surfaces is the cause of the presenes cf water in the atmosphere, whether in the form of invisible vapour or of cloud, mist, fog, rain, hail, or snow. The cooling caused by evaporation is applied in industry in the case of the ammonia compre ion machine, in working refrigerating plant for the preservation of food-stutis, and in the manufacture of artificial ice. When evaporation takes place at the surface of a dry solid, the proce is called sublimation, and it may be observed a the cases of iodine, camphor, and ce.