40 Mr. Meikle on Specific and Latent Heat, 



of saturated steam increases in a slower ratio than its elastic 

 force. The experiments of Mr. Dalton and M. Gay-Lussac 

 have completely settled this point. 



But though the vaporization of water in vacuo require less 

 heat as the temperature is higher, I do not suppose this to hold 

 when the process goes on under the pressure of another elas- 

 tic fluid, as for instance, the atmosphere. For I presume, an 

 ounce of moisture dissolved in the air will, under the same 

 barometric pressure, possess nearly the same latent heat, how- 

 ever much it may be diffused through the atmosphere, or 

 whatever be the temperature ; and that this latent heat will in- 

 crease while the barometric pressure diminishes, and vice 

 versa. From analogy, it is not improbable it will yet turn out 

 that the specific heat of aqueous vapour is to that of air in 

 which it is mixed, as 8 to 5. 



When an elastic fluid is compressed without liquefaction, 

 its capacity for heat diminishes, but increases by dilatation ; 

 and it has never been shown in what degree this affects the 

 latent or specific heats, though it is more likely the change 

 takes place chiefly in the former. Every gas has no doubt its 

 latent heat, as well as vapours. 



Throughout their whole range, the experiments of Messrs.' 

 Dalton, Ure, and Taylor* show, that whilst the temperature 

 increases uniformly, both the force and density of saturated 

 steam increase slower \ than in geometrical progression; but 

 if the experiment of M. Clement be correct, as mentioned by 

 M. Poisson, Annales de Chimie, torn, xxiii., the reverse takes 

 place at high temperatures. Thus he found the force equal 35 

 atmospheres at 215° Centigrade, or 419° Fahr. ; whereas, had 

 it followed the same rate of increase as we should have been 

 led to expect from the former experimenters, the force would 

 only have been about half that quantity. I suppose the tem- 

 perature is by an air-thermometer; but still there is something 

 unexpected in the result. The density of such steam will be 

 only 26*64' times as great as at 212°, and not 35 times, as 

 many erroneously suppose. 



* See Phil. Mag. vol. lx. p. 452. 



•J- In theEdinb. Encycl. article ''Meteorology," it is inadvertently stated, 

 that this increase is faster than in geometrical progression. On the slightest 

 reflection, however, it is obvious that the illustration there adduced from 

 Mr. Dalton, just proves what I have stated above. 



The discordance in the results of different experimenters at high tempe- 

 ratures is very likely owing in a great measure to differences in their ther- 

 mometers. The vapour of mercury may also have operated more in some 

 cases than in others. It seems owing to such vapours that barometers in 

 which no air can be detected are depressed by heat, though a correction 

 be usually applied quite in the contrary way, and of course to magnify the 

 error. 



It 



