Youne— Vapour-pressure of a pure Liquid at constant Temp. 95 
process; and, unless much time is allowed before each reading 
is taken, the temperature and, therefore, the vapour- pressure 
will be too high, except at the first reading at the largest 
volume; the error also will tend to accumulate at successive 
readings. But if much time is allowed, the error due to vapori- 
sation of mercury must increase, and that due to traces of air or 
other impurity may, and usually will, also increase. On all 
grounds, then, it is better to commence the readings of vapour-. 
pressure at the smallest and not at the largest volume. 
4,5, and 6. As the errors coming under the last three head- 
ings may be in either direction, they do not specially affect the 
question under discussion, although they are of importance when 
the actual vapour-pressure of a liquid at a given temperature is 
being considered. 
Battelli and others contend that the vapour-pressure of a pure 
liquid rises when the volume of vapour, relatively to that of the 
liquid, is diminished. If that were really the case, it would, of 
course, make no difference in what manner the alteration in the 
relative volumes of vapour and liquid was brought about, whether 
by condensation, by evaporation, or by removal of part of the 
vapour or liquid. For example, imagine a barometer tube of the 
form shown in the figure. Let there be a short column of a pure 
liquid over the mercury, but below the stopcock, the whole space 
above the mercury containing nothing but the pure liquid and its 
