THE BRISTOL PERIOD 75 



determine the vapour pressures of the other substances ourselves, 

 and for this purpose we employed our new method already 

 described [see also Trans. Chem. Soc. 47, 42 (1885)], and we also 

 made a few determinations with an air thermometer to obtain 

 fixed air-temperature points on our scale. 



The pressures were plotted in each case against the temperature 

 and were then read off for each degree over rather a wider range 

 than that actually required. I undertook to smooth these pres- 

 sures by the method of differences, and in doing this I noticed : 



(1) That in the case of chlorobenzene and bromobenzene the 



value of -JT-. T at any given pressure was the same for both 

 substances, and that with the other substances, excepting per- 

 haps ethyl alcohol, the values of -J* T at any given pressure 



showed only moderate differences. I found also that the relative 

 values were about the same whatever the pressure chosen. 



(2) The ratio of the absolute temperature of bromobenzene 

 to that of chlorobenzene was the same at all pressures, taking 

 always the same pressure for both substances, and I found after- 

 wards that this is the case also for other pairs of chemically 

 closely related substances; or R'=R, where R' is the ratio of 

 the absolute temperatures at a pressure p' and R is the ratio at 

 a pressure p. 



I had at this time secured a copy of Regnault's complete 

 researches, and I made a thorough study of his determinations 

 of vapour pressure and thus obtained confirmation of the above 

 generalisations. In a joint paper, read before the British Associ- 

 ation in 1885 at Aberdeen, Eamsay gave an account of the closely 



allied generalisation, = constant (approximate) at the same 



v i~ v z 



pressure, and I described the above generalisations (1) and (2). 



For less closely related substances the simple relation R' =R 

 does not usually hold, but I found the equation R' =R + c (t r - 1) 



