2 Mr. J. H. Alexander on the Tension of Vapour of Water. 



grees) which expresses the existing temperature. Or, what 

 amounts to the same thing, the pressure of steam whose tem- 

 perature is observed on any scale, is directly as the number 

 of degrees read for the temperature; and inversely, as the 

 whole number of degrees on the same scale between the 

 melting of ice and the boiling of water. With Fahrenheit's 

 scale, calling t the number of degrees at any temperature, the 

 pressure of steam at that temperature must be proportionate to 



. A<yain, the pressure of steam must be always directly as 



180 & » f J J 



the absolute heat of conversion, or, as it is otherwise termed, 

 the latent heat ; expressed, of course, in degrees of the scale 

 assumed. For, the greater the number of degrees for such 

 latent heat, the greater also will be the repulsive force of the 

 heat existing in the steam ; which repulsive force may be as- 

 sumed to be at least a function of the elasticity of the vapour. 

 And as such repulsive force takes effect in part by expanding 

 the medium vaporized, and the greater such expansion, the 

 less will be the remaining elasticity, it follows that the pres- 

 sure of steam in the ordinary state of an atmosphere must be 

 also inversely as its increased volume. This increased volume 

 may be taken, from the experiments of Guy-Lussac, at 1695 

 times that of water at its greatest density. And the latent 

 heat of steam is generally admitted as 990° F. ; which number 

 results from the experiments of Clement and Desormes, is 

 not far from the mean of several other observers, and will pro- 

 bably require a very small modification only to be identical with 

 the deduction from a strict theory of volumes applied to vapours 

 generally, in the mechanical relation of the observed effects of 

 heat upon such vapours and the liquids producing them. 

 So far as we have gone, then, the pressure of steam must be 



/ t 990 \ T • . 11 . • • 



I 7777:+ ,^^1 )• It IS here that the empu'icism comes m, 



\loO io95/ 



and dictates the numerical power to which this ratio must be 

 involved, in order to harmonize the progression of the elas- 

 ticities with the series of the temperature. That the simple 

 ratio will not present such harmony is manifest ; for that would 

 be the measurement, by a lineal scale of equal parts, of central 

 forces, which, acting upon volumes, might rather be supposed 

 to be in the duplicate ratio of the cube. Such a ratip, equi- 

 valent to the sixth power, is in fact what has been taken ; and 

 the complete equation becomes then, calling p the pressure, 



\180 1695/ 

 This index, however plausible upon reflection the reason to 

 justify its adoption might have appeared, was no doubt sug- 



as 



