Youne— Vapour-pressure of a pure Liquid at constant Temp. 103 
and refilled. On two occasions, however, readings were taken at 
a few temperatures, but were not made use of. ‘The pressures were 
from 0-1 to 1:0 per cent. higher than were afterwards observed 
when the tubes were refilled, and the mean value of A for the 
eight determinations was 0°53, which is even higher than for any 
of the substances which had undergone partial decomposition. 
Group LY. 
The experiments with acetic acid do not throw much light on 
the question, and need not be discussed. 
READINGS TAKEN DURING CONDENSATION. 
A few observations were made both during evaporation and 
during condensation, and a comparison of the results obtained 
by the two processes is, therefore, possible. 
1. Stable Substances. 
The mean percentage changes in pressure during condensation, 
A,, and during evaporation, A., and the differences between them, 
A. — A., are given below, with the number of determinations, J, 
up to and above 180°. 
Temperatures, N Ac Ac A.—Ae 
up to 180°, . 22 + 0:206 + 0:065 + 0°141 
above 180°, . 11 + 0°269 + 0:044 + 0°225 
The individual differences, A, — A., were positive in every case 
but one, that of normal hexane at 80°, for which both A, and 
Ay Si 
If the very slight fall in pressure frequently observed during 
evaporation were attributable to a real dependence of vapour- 
pressure on volume, the rise of pressure during condensation should 
be numerically equal to the fall during evaporation, except for 
errors in reading and slight differences which might be caused by 
taking readings too quickly. If, on the other hand, the change in 
pressure is due to the presence of air, mercury-vapour, or other 
impurity, then, as has been pointed out, the rise in pressure during 
condensation should be greater than the fall during evaporation. 
