96 Scientific Proceedings, Royal Dublin Society. 
vapour; and let the temperature be kept constant by means of a 
suitable bath. Let the height of the mercury be first read with the 
stopcock open, when the volume of vapour is very large relatively 
to that of the liquid. Then let the stopcock be closed, so as to shut 
off the bulb. The volume of vapour will now be relatively small, 
and, according to Battelli, the vapour-pressure should therefore be 
higher. A depression of the mercury should in that case be 
observed. 
That such a result would actually be obtained seems to me 
to be inconceivable; at any rate, there must, I think, be ample 
experimental proof of the variability of vapour-pressure before the 
formula p = ¢ at constant temperature is rejected. 
It is only by the elimination of the errors to which refe- 
rence has been made that the true relation between vapour- 
pressure and volume can be ascertained; and if variable pres- 
sures are observed, clear proof of the absence of such errors 
must be afforded. If, on the other hand, it can be shown that 
the more completely the errors are avoided or eliminated, the 
smaller is the variability of vapour-pressure with volume; and if, 
under the most favourable conditions, the variation is found to 
be negligible, then it may be concluded that satisfactory experi- 
mental evidence of the constancy of vapour-pressure has been 
obtained. 
In studying the available experimental data, the following 
points should specially be noted :-— 
1. For the various reasons given, better results must have been 
obtained during evaporation than during condensation. 
2. The errors are likely to be greater at high temperatures 
than at low ones. 
. If any signs of decomposition have been observed, the 
pressures are likely to be somewhat higher at small 
volumes than at large. 
4, If there are two series of determinations of the vapour- 
pressures of a stable liquid, and a rise of pressure during 
condensation (or a fall during evaporation) is observed 
in the one series (A), but not in the other (B), and if 
the mean pressures in series A are higher than those in 
B, it may be concluded that air was not completely 
removed from the liquid in series A. 
(os) 
