Young — Vapour-Pressures, ^c, of Thirty Pure Substances. 377 



saturated vapours of all the substauces investigated, witli the single exception 

 of acetic acid, behave more and more like perfect gases as the temperature 

 falls, although the real vapour densities under normal pressure are in every 

 ease higher than those calculated on the assumption that Boyle's and Gray- 

 Lussao's laws are strictly true. 



In order to discover whether there was a serious error in the trend of 

 any of the curves, the following device was adopted : — The volumes of a 

 gram of vapour were calculated from tlie observed pressures for a range of 

 50 or 60 degrees from the boiling-point downwards on the assumption — 

 (1) that the vapour behaved as a perfect gas, and (2) that the ratio of the 

 actual to the theoretical density was constant and equal to that actually 

 observed under a pressure of about two atmospheres (generally about 1'07 to 

 1-10). 



Tiie logarithms of these tlieoretical volumes were plotted against the 

 temperatures, as were also the logarithms of the observed volumes for a 

 range of 100 or 120 degrees, that is to say, from about the boiling-point 

 under normal pressure to a temperature about 100 degrees higlier. 



It is evident that the curve for the real vapour should, if produced, lie 

 between the two theoretical curves, and that, with falling temperature, it 

 should approach that for a perfect gas. 



In this way it was found that, in drawing the original curves, too much 

 weight had, in several cases, been attached to the least accurate observations 

 at tlie lowest temperatures, but that, on the other hand, the results witli 

 iodobeuzeue at the lowest temperatures were more accurate than had been 

 supposed. In most cases the reconstructed curves differed only slightly 

 from those originally drawn. 



It was then found that the particular data required could best be 

 obtained by plotting the logarithms of the densities of saturated vapours 

 (water at 4° = 1) against the logarithms of the pressures, guiding curves 

 being drawn as before. The curves for the real vapours were found to 

 approximate very closely indeed to straight lines at and near the normal 

 pressure, so that the following formulae could be employed : 



log Sp = A + a log;;, (1) 



(where s is the density and p the pressure). The values of A, a, s, and 



ds 



— under normal pressure have been published in a separate paper.' 



' Journ. de I'hysique, (4), viii., p. 6, 1909. 



