48 Preservation of Animal Food. 



the freezing point the atmosphere of a room or of a ship's 

 hold, and this at an expense not very much more than would 

 be entailed by communicating a quantity of heat to the air 

 reduced in temperature, equal to that which had been 

 abstracted by the means alluded to above. 



That a change in volume is always accompanied by a 

 change in capacity, for heat is the law I employ to effect this 

 change in the temperature of air. Theoretically, in selecting 

 any gas as a medium to utilise this law, I should choose one 

 possessing a high specific heat, as on compression from equal 

 volumes to equal volumes, the latent heat rendered sensible 

 in any one gas, is to the latent heat rendered sensible in any 

 other, as the specific heat of the one is to the specific 

 heat of the other. But for practical purposes, when 

 the object to be attained is that stated above, I 

 employ the atmosphere, it having the advantage of being 

 always obtainable, and of not possessing any injurious 

 qualities, as almost every other gas does in a greater or a less 

 degree. 



The conduct of air in expanding is marked by a certain 

 peculiarity that, at first, appears somewhat perplexing, yet, 

 on investigation, this seeming inharmonious department is 

 discovered to be in consonance with one of nature's grandest 

 laws — the indestructibility of force. I refer to the fact that 

 air, on expanding from a certain volume in an elastic medium, 

 does not apparently evince a capacity for heat equal in quan- 

 tity to the heat developed by its compression to that volume 

 from a volume equal to that to which it expands. You will 

 observe that I use the word apparently, for, in accordance 

 with the law previously mentioned, its capacity is exactly 

 equal. To explain this result, it must be remembered that 

 the atoms of air in expanding acquire momenta, which 

 represent quantities of heat ; when these momenta are 

 destroyed, the heat which they represent is rendered appa- 

 rent, and assists in diminishing the fall of temperature 

 due to the sensible heat of the air being converted into 

 latent heat by its expansion. This is a phenomenon of the 

 utmost importance, and should be prominently kept in view 

 in all attempts to obtain cold by the expansion of gases. 

 Several comparatively unsuccessful attempts (including - the 

 inventions of two distinguished philosophers) to produce 

 cold by the expansion of highly compressed air, are due to a 

 misapprehension on this point. Sir John Herschel] says that 

 " An old steam-boiler buried some twenty or thirty feet 



