1869.] of Ice and Cold. 201 



Since the tension of a vapour at any temperature is the measure 

 of the facihty with which the liquid evaporates at that temperatm'e, 

 it will be seen from the data in this table that, in this respect, there 

 is a very considerable diflerence between the liquids there named. 

 Here, again, the characters of hquid ammonia are such as to give 

 it a marked precedence over all the other liquids, as a refrigerating 

 agent, by reason of its relative capability of vaporizing at very 

 low temperatures. This substance is in fact gaseous under normal 

 l)ressure, within the ordinary range of atmospheric temperatm-e, the 

 boiling of the liquid being many degrees below the zero of Fahren- 

 heit's scale; and at ordinary temperatui'es it requires a pressure of 

 from eight to ten atmospheres — 117 to 150 lbs. per square uich — 

 to maintain it in the liquid state. 



Alcohol, although it has a greater capability than ether of ab- 

 sorbing heat in vaporizing, is still inferior to ether as a refrigerating 

 agent, on account of its being much less readily vaporized at low 

 temperatures ; and even ether evaporates so slowly at temperatures 

 much below its normal boiling-point, that it can be used for refrige- 

 lating only with the aid of an air-pump to maintain the requisite 

 rate of vaporization. 



Liquid ammonia is therefore by far the most efficient material 

 to use for this purpose, not only on account of its ready vaporiza- 

 tion at low tempei-atures, but also because its power of absorbing 

 heat in that change is but little inferior to that of water. 



Another process, in which heat is expended and rendered latent, 

 is the expansion of air. The amount of heat thus absorbed is at 

 the rate of '069, or about T^tli of a heat unit for each pound of 

 air expanded to the extent of •002035, or about ^T,uth of its 

 volume at 32^ Fahr. under normal pressure. If therefore air be 

 compressed, say to one-tenth of its bulk, and, after being cooled 

 to a low temperature, it be allowed to expand in such a way as to 

 perform mechanical work, such as moving a piston, there is an 

 expenditure of heat proportional to the resistance overcome and to 

 the degree of exj^ansion. Consequently the temperature of the gas 

 is reduced during the act of expansion, and this effect may be taken 

 advantage of for purposes of refrigeration. The chief disadvantage 

 of this method consists in the great expenditure of power requisite 

 for compressing the air, which involves a large consumption of 

 fuel. 



From what has been stated, it will be apparent that at present 

 the choice of a refrigerating agent for producing ice or great 

 degrees of cold, lies between ammonia, ether, and air, and that 

 ammonia presents tlxe greatest advantages for this purpose. 



The expansion of compressed air appears to have been the 

 means first adopted for making ice, by Dr. Gorrie of America; and 

 in this country ether was the material employed in one of the 



