200 The Artificial Production [April, 



The amount of heat thus disposed of and rendered latent in the 

 formation of steam from water is considerably greater than that 

 existing in the latent condition in liquid water, or, what amounts to 

 the same thing, that expended in melting ice ; but the vaporization 

 of water cannot be applied as a means of refrigeration to any great 

 extent, because under the ordinary atmospheric pressure it does not 

 take place readily or with sufficient rapidity at temperatures much 

 below the normal boiling point, or 212^ Fahr., and even when the 

 pressure is removed by means of an air-pump, the vaporization of 

 water proceeds very slowly at low temperatures. There are, 

 however, other substances which vaporize readily under these 

 conditions: and, for this reason, they are specially suited for 

 artificial refi'igeration, although the amounts of heat expended and 

 rendered latent in their vaporization are less than in the case of 

 water. Ether, alcohol, and liquid ammonia are substances of this 

 kind ; and, according to the foregoing data, expressing the latent 

 heat of their vapours, the quantities of each of these substances 

 which would have to be vaporized, in order to produce a ton of ice 

 from water at 60° Fahr., or to produce a refrigeration equivalent 

 to the melting of a ton of ice, would be :— 



lbs. 



Ether 2,348-009 



Alcohol 1,049-272 



Liquid ammonia 424-728 



From this comparison it mil be seen that the expenditure of 

 heat accompanying the vaporization of liquid ammonia is much 

 greater than it is in the case of alcohol or ether, and that in this 

 respect it is the most powerful as a refrigerating agent. But the 

 amount of heat rendered latent in the vaporization of any substance 

 is not the only, or even the chief point which determines its efficiency 

 as a refrigerating agent. The degree of facility with which a 

 substance vaporizes at low temperatures is of still gi'cater import- 

 ance, as will be evident ii-om the following table, which gives the 

 tension of the vapours at difi'erent temperatures below the boiling- 

 points of the liquids under normal atmospheric pressure : — 



Ammonia. 



Water. 



