Youne— Vapour-pressure of a pure Liquid at constant Temp. 97 
The three substances for which the data are most complete are 
isopentane, normal pentane, and normal hexane. All three hydro- 
carbons are perfectly stable, and the critical temperatures of the 
first two are so low that no appreciable error can have been caused 
by vaporisation of mercury. 
In the opposite Table, details of the experimental faults 
obtained with normal pentane are given, the data including the 
lowest and highest ratios of the volume of vapour to that of 
liquid (Voap,/ Vig.) at each temperature, the corresponding vapour- 
pressures, the difference in mm., and the percentage difference 
between the observed pressures. 
The mean percentage difference in pressure for the 38 determi- 
nations is — 0:005, whereas, according to Battelli, there should 
have been a marked positive difference. 
Three different specimens of isopentane were prepared and 
examined, and there were altogether 59 determinations of vapour- 
pressure. Here, again, the mean percentage difference in pressure, 
— 0-007, was quite negligible, the apparent slight rise of pressure 
with expansion being, of course, due to experimental error. 
With normal hexane the mean percentage difference for 
82 determinations (two specimens having been examined) was 
+ 0-074, a value which, though positive and larger, may be re- 
garded as within the limits of experimental error, an equally 
large negative value having been observed with di-isopropyl. 
A full account of the experimental results, with a detailed dis- 
cussion of the conclusions to be drawn from them, and of the 
influence of the various errors to which reference has been made 
in this paper, will be published in the Journal de Chimie Physique. 
It will be sufficient here to give the mean results obtained with 
each substance. 
In several cases the results were not quite satisfactory, owing 
either to incomplete removal of air or to partial decomposition of 
the liquid investigated. ‘The substances (or rather the series, for 
the same substance may appear in more than one group) are 
classified as follows :— 
I. Series in which no signs of decomposition were observed, 
and in which there was no evidence of more than a trace 
of air being present. 
