104 Scientific Proceedings, Royal Dublin Society. 
The fact that the rise was always (with the one exception) greater 
than the fall, therefore, affords evidence that the small positive 
values of A, were due to the presence of minute quantities of air 
or mereury vapour. 
2. Substances which had undergone some Deconyposition. 
Temperatures, N Ac Ae Na = (Np 
UO wo WIT, 5 Le + 0°57 + 0°22 + 0°35 
Moonie) Ise, 2 | AAI + 0°96 + 0:29 + 0°67 
As might be expected, the values of A,, A., and A, — Az, are 
all much higher than for stable substances, and the influence of 
temperature is very marked. 
The formula p = c at constant temperature is not only of great 
intrinsic importance as an equation of state, but it has indirectly 
an important bearing on much of the experimental work that has 
been carried out on substances in the neighbourhood of their 
critical points. As is well known, many of the results obtained 
by different observers are contradictory, and are not in agreement 
with the views expressed by Andrews. Now, if the equation p =¢ 
is really true—and in my opinion the experimental evidence 
brought forward in this paper affords ample proof that it is 
true—the variability of the vapour-pressure of any liquid during 
evaporation or condensation at moderate temperatures must 
be taken as a proof that that liquid contains air or other 
impurity (assuming, of course, that correct methods of heating, 
&e., are employed). But if the liquid is not free from 
such impurity, the phenomena observed near the critical point 
must, of necessity, be misleading. In my own work—and my 
- results have consistently been in agreement with the views of 
Andrews—I have always regarded variability of vapour-pressure 
as a sure indication of the presence of impurity; and when such 
variability has been observed to any marked extent, I have dis- 
continued the determinations and refilled the experimental tube. 
The results described in this paper show, I think, that in the ease 
of a stable liquid, constancy of vapour-pressure at moderate 
temperatures is always attainable. 
