432 Proceedings of Poycd Society of Edinburgh. [sess. 
liquids in the two tubes, with proper correction for the densities of 
the two liquids at their actual temperatures in different parts of 
their columns, gives the difference of vapour pressures for the 
actual temperatures of the two liquids at their upper surfaces. 
§ 9. To facilitate and approximately determine the hydrostatic 
correction for specific gravities at the actual temperatures of the 
two liquids, open wide the stopcock above the top of one of the 
two glass tubes, and let a little air run back from the air-pump, by 
very cautiously and slightly opening our upper air-pump stopcock, 
and closing it again before the liquid surface reaches the lower 
end of its glass tube. Then open wide the stopcock over the top 
of the other glass tube. After that, by cautiously opening and 
closing our lower air-pump stopcock, let in a little air to the bottles 
until the mean level of the liquids in the two columns rises to 
nearly the same level as it had in the observed positions of § 8. 
In the present circumstances, air in the upper bent metal tube 
resists diffusion of vapour through it sufficiently to prevent any 
important difference of temperatures from being produced by 
evaporation and condensation at the two liquid surfaces, and there 
is practically perfect hydrostatic equilibrium of equal liquid pres- 
sures at the tops of the two columns. 
§ 10. The vapour pressure of water is accurately known through 
a very wide range of temperature from Regnault’s experiments; 
hence, if pure water be taken for one of our two liquids, the mode 
of experiment described above determines the vapour pressure of 
the other liquid. 
§ 11. The apparatus may be kept day after day with the same 
liquids in it (all the stopcocks to be closed, except when it is not 
in use for observations) ; and thus the observations for difference 
of vapour pressures may be repeated day after day ; or a long 
series of observations may very easily be made to determine vapour 
pressures at different temperatures. Always before commencing 
observations the operation of § 7 must be repeated to remove air 
or other volatile impurity, if any has escaped from dissolution in 
either liquid into the vapour space above it, or if any air has 
leaked in by the upper stopcocks. 
