172 



EVAPORATION. 



current of air passing over their surface. In cases where it has been found 

 convenient to promote the evaporation by heating the liquid, the heat is fre- 

 quently applied only to the surface, instead of being communicated by fire at 

 the bottom of the vessel. In fact, the current of air which is made to pass 

 over the surface of the evaporating liquid, is previously heated by forcing it 

 through a fire. The flame of the fire is also, sometimes, made to play over the 

 evaporating surface. 



The coolers in breweries are large shallow vessels, exposing a considerable 

 surface with a small depth of the liquid. They are commonly placed at the 

 top of the building, and are open on every side to the air, so that in whatever 

 direction a wind blows a current of air must pass over them. There are also 

 provided a number of revolving fans, by which the stream of air in immediate 

 contact with the evaporating surface, is continually kept in a state of agitation. 

 The evaporation has a continual tendency to saturate the stratum of air im- 

 mediately over the liquid, and by these expedients this stratum is caused to 

 undergo a constant change ; the air saturated with vapor being driven away, and 

 a fresh portion supplying its place. 



•When salt is held in solution by water, the process of evaporation affects 

 only the water, and loosens the connexion produced by the affinity of its par- 

 ticles for the molecules of the salt. If the solution, in this case, be what is 

 called a saturated solution, that is, if it contain as much salt as the water at the 

 given temperature is capable of sustaining, then the least quantity of evapora- 

 tion must be attended with a deposition of crystals of salt in the liquid ; and, 

 if the evaporation be continued, the water will, at length, altogether disappear, 

 and nothing but a mass of crystallized salt will remain. 



This principle forms the basis of the method by which salt is obtained from 

 sea-water. The water is received into a number of large shallow ponds, lined 

 with clay, and prepared on the seashore. The water, being received into 

 these, and dammed in, is left exposed to the weather in the heat of summer. 

 If the weather be dry, the quantity of evaporation will considerably exceed 

 the quantity of rain, and large surfaces being exposed in proportion to the 

 depth of water in the pits, the water will be gradually dissipated, and will 

 at length, altogether disappear, and a quantity of what is called bay salt will 

 remain behind. This salt is said to be the fittest for the purpose of curing 

 fish. 



When ice cannot be obtained, wine may be cooled in various ways by the 

 process of evaporation. If a moist towel be wrapped round a decanter of 

 wine and exposed to the sun, the towel in the process of drying will cool the 

 wine ; for the wine must supply a part of the latent heat carried off by the va- 

 por in the process of drying the towel. Wine-coolers constructed of porous 

 earthern ware act on a similar principle. The evaporation of water from the 

 porous material reduces the temperature of the liquid immediately surrounding 

 the wine. Travellers in the Arabian deserts keep the water cool by wrapping 

 the jars with linen cloths which are kept constantly moist. 



Historians mention that the Egyptians applied the same principle to cool 

 water for domestic purposes. Pitchers containing the water were kept con- 

 stantly wet on the exterior surface during the night, and in the morning were 

 surrounded by straw to intercept the communication of heat from the external 

 air. 



In India the curtains which surround beds are sprinkled with water, by the 

 evaporation of which the air within the curtains is cooled. 



The absorption of heat in evaporation will enable us easily to comprehend 

 the danger arising from wearing damp clothes, or from sleeping in a damp bed. 

 In the animal economy there is a source, the nature and operation of which is 



