=174. Cooling Effect of Evaporation.=--The molecules that leave an evaporating liquid are naturally the swiftest moving ones, that is, the ones having the highest temperature, so their escape leaves the liquids cooler than before, and the one whose molecules leave fastest is naturally the one that becomes coldest, that is, the ether, in the experiment of Art. 173. If no air pressure were exerted upon the surface of the liquid, the escape of the molecules would be much increased and the temperature of the liquid would be lowered rapidly. To test this, fill a thin watch glass with ether and place it over a thin slip of glass with a drop of cold water between the two. Now place this apparatus under the receiver of an air pump and exhaust the air. The rapid evaporation of the ether so lowers its temperature, that often the drop of water is frozen. The lowest temperatures are obtained by evaporating liquids at reduced pressure. Onnes by evaporating liquid helium at a pressure of about 1.2 mm. reached the lowest temperature yet attained, -456°F., or -271.3°C. If four thermometers are taken, the bulbs of three being wetted respectively with ether, alcohol, and water the fourth being dry, on vigorously fanning these, the moistened thermometers show that they have been cooled while the dry one is unaffected. This indicates that fanning a dry body at the temperature of the air does not change its temperature. Fanning does increase evaporation by removing the air containing the evaporated molecules near the surface of the liquid so that unsaturated air is continually over the liquid. If a pint of water is placed in a bottle and another pint in a wide pan the latter will become dry much sooner because of the greater surface over which evaporation can take place. Application of this is made at salt works where the brine is spread out in shallow pans. =175. Rate of Evaporation.=--The rate of evaporation is affected by several factors. These have been illustrated in the preceding paragraphs. To briefly summarize: The rate of evaporation of a liquid is affected by-- (a) The nature of the liquid. (b) The temperature of the liquid. (c) The pressure upon its evaporating surface. (d) The degree of saturation of the space into which the liquid is evaporating. (e) The rate of circulation of air over its surface. (f) The extent of surface exposed to evaporation.