1869.] of Ice and Cold. 199 



it is brought in contact with, which has a temperature higher than 

 82^ Fahr. Hence it will be seen that refrigeration, or the produc- 

 tion of cold, is simjily a manipulation of heat. It is an operation 

 in this respect perfectly analogous to the production of a high 

 temperature, in so far as both processes consist in the transfer of 

 heat from one substance to another, and are subject to the same 

 general laws. They are however reverse processes. Thus in 

 generating steam, heat produced by the combustion of fuel is 

 communicated to water. In making ice, on the contrary, heat 

 is abstracted from water, and in this process the water which is 

 cooled corresponds to the fuel burnt in generating steam, or in 

 converting any other substance into vapour. Just in the same 

 way that the fuel in burning yields its heat to the substance 

 vaporized, so does the water, in making ice, yield its heat to some 

 other substance capable of receiving it. 



This is the nature of the work to be done in making ice, and it 

 is now necessary to consider the amount of that work requisite for 

 producing a given quantity of ice. 



Water at the temperature of 60^ Fahr. contains an amount of 

 heat greater than that contained in an equal weight of ice at 

 32° Fahr. to the extent of 170 "65 heat units* for each j)ound, 

 consequently to convert water at 60° Fahr. mto ice, it is necessary 

 to abstract that amount of heat from it. Thus to jDroduce a ton 

 of ice the quantity of heat to be abstracted from water at 60° Fahr. 

 amounts to 



Heat units. lbs. 

 382,256 = 2240 x 170-65. 



This is a quantity of heat not more than about one-eightieth 

 part of that capable of being generated by the combustion of a ton 

 of ordinary coal. 



The means by which this amount of heat may be abstracted 

 from water consist in producing some physical change involving an 

 expenditure of heat, and doing this in such a way that the heat 

 required for, and applied to that purpose, is abstracted from the 

 water to be cooled and frozen. The conversion of any substance 

 into vapour is a change of this kind, which involves an expenditure 

 of heat similar to that taking place in the melting of ice. The 

 amounts of heat thus absorbed by various substances in vaporizing 

 are as follows : — 



Latent heat per lb. 



Heal units. Authority. 



Water 966-1 .. ., Eognault. 



Liquid ammonia .. .. 900-0 .. .. Favre uml Silbermaiin. 



Alcohol 364-3 .. .. ) . , 



Ether 162-8 .. .. /^ndn 



ews. 



* The unit of heat here referred to is the quantity of heat required to raise the 

 temperature of a pound of water from 40° to 41" Fahr., or one degree of tem- 

 perature. 



p 2 



