170 



EVAPORATION. 



some constituent part of the mixture which it is important to preserve. For 

 this reason, the above method is said to have been adopted with advantage in 

 the distillation of vinegar, which it is impossible to distil in the ordinary way 

 without giving it a peculiar burnt flavor ; but by distilling it in vacuo, the vapor 

 is raised at the temperature of 130°, and this effect is avoided. 



In the process of sugar refining it was found that by raising the syrup to 

 the necessary temperature, a risk was incurred of burning or decomposing it 

 by too much heat. The method of boiling in vacuo was adopted by Mr. Ed 

 ward Howard to remove this inconvenience. The syrup is thus concentra 

 ted to the granulating point without risk of decomposition. This method is 

 now generally followed. 



When vapor was produced from a liquid by ebullition, we have observed 

 that a large quantity of heat was absorbed in the transition from the liquid to 

 the gaseous form. The same effect attends the production of vapor from the 

 surface, and, in fact, it is an indispensable consequence of the transition of a 

 body into the vaporous form, at whatever temperature that transition takes 

 place. In the formation of vapor, therefore, a quantity of heat must be supplied 

 to the vapor formed, and must become latent in it ; and this heat must be sup 

 plied either by the body itself or by surrounding objects. By whatever means 

 it is supplied, the object which communicates it must undergo a corresponding 

 depression of temperature ; and hence vaporization becomes a means for the 

 production of cold, on a principle precisely analogous to that of freezing mix 

 tures. 



This principle is illustrated by the method used to cool water for domestic 

 purposes in hot countries. The water is placed in certain porous vessels, 

 called in the East alcarrazas, and these are suspended in a current of air : as, 

 for example, between two open doors. The vessel allows the water to pene 

 trate it, and thus exposes it more effectually to evaporation, as well from the 

 surface of the liquid itself, as from the exterior surface of the vessel containing 

 it. As the vapor is formed, a quantity of latent heat is necessary for it ; and 

 this latent heat is supplied from the water contained in the vessel, which un 

 dergoes a corresponding depression of temperature. 



The same effect can be made manifest by surrounding the bulb of a ther 

 mometer by a moist sponge, and exposing it to the sun. Let another ther- 

 mometer be at the same time placed near it in the shade, and the thermometer 

 surrounded by the sponge Will be observed rapidly to fall, while the thermom- 

 eter in its immediate neighborhood is stationary. This effect is evidently pro 

 duced by the rapid evaporation of the water with which the sponge is saturated, 

 and a corresponding depression of temperature produced in the liquid remain 

 ing in the sponge, arising from the heat supplied by it to the vapor. 



The depression of temperature produced by evaporation will be more per- 

 ceptible the more rapid is the evaporation, because then the body from which 

 the heat is abstracted has not time to receive a supply of heat from surrounding 

 objects to replace that which it has given out. Hence, by conducting the pro 

 cess of evaporation in a vacuum, where the evaporation is almost instantaneous, 

 the cooling effect is more conspicuous. If a quantity of water included in the 

 bulb of a thermometer tube be surrounded with a sponge moistened with ether, 

 and placed under the receiver of an air-pump, the moment the air is withdrawn 

 the ether suddenly evaporates ; and if a sufficient quantity of ether be supplied, 

 the water in the bulb will be frozen. 



The same fact may be exhibited in a still more striking manner, by pouring 

 some ether on the surface of water in a flat vessel. When the receiver placed 

 over these is exhausted, the ether will boil in consequence of the removal of 

 the atmospheric pressure, and its rapid evaporation will presently cause the 



