ARTIFICIAL REFRIGERATION AND ICE MAKING, XLVII. 
specific heat of the medium multiplied by the number of degrees 
it is lowered in temperature. All this cooling power is absolutely 
lost because the medium has to be heated up again by the expendi- 
ture of cnergy at every circuit it makes through the machine. 
THREE PROPERTIES OF A GAS CONCERNED IN FORMING AN EFFICIENT 
REFRIGERATING MEDIUM. 
From the foregoing remarks it will be understood that the 
efficiency of different gases for refrigerating purposes is mainly 
dependent upon three properties possessed by them, and not upon 
any one special quality, and these are— 
1. A low temperatue of vaporization upon which depends the 
degree of cold that can be produced by such evaporation. 
2. A high latent heat upon which depends the total number of 
heat units which will be abstracted by the evaporation of a given 
weight of the medium. 
3. A low specific heat upon which depends the percentage of 
the heat taken up by (2) or the amount of cold produced which 
can be utilised. : 
WHY AMMONIA IS SO LARGELY USED IN REFRIGERATING MACHINES. 
Although ether, chloride of methyl and several other media 
have been used in refrigerating machines besides those already 
referred to, and some are still advocated under special conditions, 
yet ammonia is now used more than all the rest put together, 
experience having proved the many advantages it possesses. The 
Principal reason why ammonia has supplanted the use of other 
liquids as the circulating medium in refrigerating machinery is 
because it has such a high latent heat of vaporization, being 555 
B.T.U. at zero, against 123 for carbonic acid, and 171 for sulphur- 
us acid, that is to say one pound of ammonia at zero Fahrenheit 
in passing from the liquid to the gaseous condition would take up 
555 thermal units, while the other liquids would take up less than 
@ third and less than a fourth respectively. There are some com- 
Pensating advantages in the case of carbonic acid on account of 
its high specific gravity which makes its heat of vaporization for 
