Evaporation, on the other hand, is slower than boiling and takes place at the surface only; it goes on at temperatures much below the boiling point, but more slowly as the liquid is colder; even ice will evaporate slowly.
46. To maintain evaporation, heat must be absorbed by the liquid exactly as if it were being converted into steam at the boiling temperature, instead of being converted into vapor at a lower temperature. If there is available no source of heat by which the liquid may be warmed to compensate for the heat taken from it by evaporation, the liquid itself will be cooled. Water evaporates slowly in air. Some other liquids, like alcohol, ether, bisulphide of carbon, gasoline, and naphtha, evaporate very rapidly. It is quite possible to produce a temperature below the freezing point of water by the rapid evaporation of gasoline, and in gasoline engines this phenomenon necessitates careful attention, since the gasoline does not readily pass into vapor when cooled much below the ordinary temperature of the air. To evaporate gasoline rapidly and continuously, heat must be added; in the gasoline engine, this heat may conveniently be taken from the hot gases exhausted from the cylinder, and a part of them caused to circulate around the device used for converting gasoline into vapor.
47. In a perfect vacuum, all volatile liquids, that is, liquids that give off vapors, pass instantly into vapor. This does not mean that, if the air in a closed vessel is completely exhausted, forming a perfect vacuum in the vessel, and then a quantity of any liquid is run into the vessel, the entire quantity of that liquid will pass into vapor. As. soon as part of the liquid has passed into vapor, the space in the vessel ceases to be a perfect vacuum, and for any given temperature there is a point reached in the density of the vapor of that liquid beyond which no further evaporation will take place. This point is called the saturation point for that vapor. For example, if a quantity of water is placed in a vacuum, a portion of the water instantly passes into vapor, and the vacuum, instead of being an absolute
