Comparison of Vapour- Temperatures at Equal Pressures. 335 



This short paper is little more than a description o£ a phe- 

 nomenon of which the writer has never seen any account 

 given elsewhere; it makes hardly any attempt to explain 

 much of it ; still it is offered in the hope that some one more 

 conversant with hydrodynamics than the author may give the 

 true solutions to the questions it suggests. 

 Eton, Bucks, May 1902. 



XXXVI. On the Comparison of Vapour- Temperatures at 

 Equal Pressures. By Professor J. D. Everett, F.R.S* 



EAMSAY and Young seem to have been the first to call 

 attention (Phil. Mag. Jan. 1886) to the fact that the 

 ratio t/t 1 of the absolute temperatures at which two vapours 

 (at saturation) have the same pressure p remains nearly 

 constant for changes of p of very considerable magnitude. 

 In the case of vapours of kindred constitution, their results 

 show that a twentyfold increase of p only changes t/t' by 

 about 3? per cent. 



They further lay down, for the comparison of vapours 

 generally, the law — now known as u Ramsay and Young's 

 law " — that if f l9 t 2 denote the absolute temperatures of one 

 vapour at the pressures p l p 2 , and tj t 2 ' those of another 

 vapour at the same pressures, we shall have 



'f-p^ih-o. ..... (i) 



i 2 i\ 



c being a small positive or negative constant multiplier, de- 

 pending on the substances compared. 



To the eye of the mathematician there is an awkward 

 one-sidedness about this formula ; it is not symmetrical as 

 between t and t'. It can, however, be rendered symmetrical 

 by first writing it in the form 



j, ~ct = k, 



(k being a constant), and then dividing by ct. We thus obtain 

 an equation of the form 



7-f-* V 



x and y standing for — k/c and 1/c, which are constants. A 

 * Communicated by the Physical Society. 



