Youne— Vapour-pressure of a pure Liquid at constant Temp. 91 
water. The pressure was corrected (1) for the temperature of the 
air in the gauges; (2) for the difference in level of the mercury 
in the experimental tube and the air-gauge ; (3) for the pressure 
of the column of unvaporised liquid ; (4) for the expansion of the 
heated column of mercury ; (5) for capillarity ; (6) for the devia- 
tion of air (in the gauge) from Boyle’s law, as determined by 
Amagat. No correction for the vapour-pressure of mercury was 
applied, because evaporation through a long column of liquid was 
found to be exceedingly slow. 
In order to obtain trustworthy results, the following sources of 
error must be taken into account, and guarded against as far as 
possible :-— 
1. The presence of impurities, including air dissolved in the 
liquid, or adhering to the walls of the tube. 
2. The vaporisation of mercury. 
3. The allowance of insufficient time for equilibrium to be 
established before taking readings. 
4, Variability of temperature of the vapour-jacket (or bath). 
5. Too rapid variation in the temperature of the water 
surrounding the air-gauges. 
6. Hrrors of reading. 
1. Impurities—Not only is the actual vapour-pressure of a 
liquid altered by dissolving some other substance in it, but—except 
in special cases where a mixture of maximum or minimum 
vapour-pressure is formed—the composition of the mixture alters 
as evaporation or condensation proceeds, the vapour being always 
richer than the liquid in the more volatile component. 
The following cases must be considered :— 
(a) The dissolved substance (impurity), present, it may be 
assumed, in very small quantity, is non-volatile. When the 
volume is increased, so that evaporation takes place at constant 
temperature, the residual solution becomes more and more concen- 
trated and the vapour-pressure falls. The change of concentration 
and of vapour-pressure with volume is slow at first, but becomes 
more and more rapid as the amount of liquid diminishes. 
(0) The dissolved substance is gaseous under ordinary con- 
ditions, and is not very soluble in the liquid. This case is the 
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