VAPORIZATION. 45 



ably by one portion, it rarely fails in being frozen when trans- 

 ferred into another portion of the mixture. For producing still 

 more intense degrees of cold, the evaporation of highly volatile 

 liquids, of fluid carbonic acid, for instance, affords the most 

 efficient means. 



VAPORIZATION. 



We have now to consider the second general effect of heat : 

 vaporization, or the conversion of solids and liquids into vapour. 

 Vapours, of which steam is the most familiar to us, are light, 

 expansible, and generally invisible gases, resembling air com- 

 pletely in their mechanical properties, while they exist, but 

 subject to be condensed into liquids or solids by cold. Water 

 undergoes a great expansion when converted into steam, a cubic 

 inch of water becoming, in ordinary circumstances, a cubic foot 

 of steam ; or more strictly, one cubic inch of water, when con- 

 verted into steam, expands into 1694 cubic inches. 



This change, like fluidity, is produced by the addition of heat 

 to the body which undergoes it. But a much larger quantity 

 of heat enters into vapours than into liquids, into steam than 

 into water. If over a steady fire a certain quantity of ice-cold 

 water requires one hour to bring it to the boiling point, it will 

 require a continuance of the same heat for five hours more to 

 boil it off entirely. Yet liquids do not become hotter after they 

 begin to boil, however long, or with whatever violence, the boil- 

 ing is continued : for if a thermometer be plunged into water, 

 and the point marked where it stands at the beginning of the 

 boiling, it will be found to rise no higher, although the boiling 

 be continued for a long time. 



This fact is of importance in domestic economy, particularly 

 in cookery ; and attention to it would save much fuel. Soup, &c. 

 made to boil in a gentle way, by the application of a moderate 

 heat, are just as hot as when they are made to boil on a strong 

 fire with the greatest violence ; when water in a boiler is once 

 brought to the boiling point, the fire may be reduced, as having 

 no farther effect in raising its temperature, and a moderate heat 

 being sufficient to preserve it. 



The steam from boiling water, when examined by the 

 thermometer, is found to be no hotter than the water itself. 

 What then becomes of all the heat which is communicated to 



