[ 89 ] 
IX. 
ON THE VAPOUR-PRESSURE OF A PURE LIQUID AT 
CONSTANT TEMPERATURE. 
By SYDNEY YOUNG, D.Sc., F.R.S., Trinity College, Dublin. 
[Read, Fesruary 20; Received for Publication, Frsruary 27 ; 
Published, May 2, 1906.] 
Iw arecent number of the Journal de Chimie Physique (19085, vol. i11., 
p- 665), there is a valuable article by Dr. P. Kohnstamm, on “ Les 
travaux récents sur l’équation d’état.’’ Kohnstamm discusses the 
part played by the Kinetic Theory, by thermo-dynamics and by 
experiment, respectively, in the development of the subject, and he 
takes a pessimistic view of the results which have been obtained by 
experimentalists. Thus, speaking of the three equations of state, 
p=c for saturated vapours at constant temperature, pv = RT for 
gases, and p (v — b) = & for liquids, he says (p. 675), “ Or, méme 
si ’on veut établir une formule, il est indubitable que les données 
expérimentales n’y suffisent pas par elles-emémes. Je passe sur ce 
qui concerne les liquides, mais ou en sommes-nous pour les vapeurs 
saturées? Onconnait un seul exemple (l’isopentane étudié par 
M.S. Young) ot Von soit parvenu, dans une série d’expériences, 
a condenser la vapeur saturée a pression constante. Dans tous les 
autres cas il s’est produit des surélévations de pression et tous les 
auteurs qui se sont laissés guider exclusivement par leurs expe- 
riences (Battelli, Wiillner et Grotrian, etc.) combattent la valeur de 
Ja formule indiquée.”’ 
It is quite true that in a short paper which I published in 
1894 in the Philosophical Magazine (vol. xxxviii., p. 569), on the 
“Influence of the relative Volumes of Liquid and Vapour on the 
Vapour-pressure of a Liquid at constant Temperature,” I referred 
especially to isopentane, because the data for this substance were 
more complete than for any other; but I stated that similar results 
had been obtained with other liquids. 
Again, I do not feel that I am justly entitled to the honour of 
being the sole champion—as an experimentalist—of the discredited 
formula. 
SCIENT. PROC. R.D.S., VOL. XI., NO. IX. N 
