1896-97.] Lord Kelvin on Vapour Pressures of Liquids. 
429 
On a Differential Method for Measuring Differences of 
Vapour Pressures of Liquids at one Temperature 
and at Different Temperatures. By Lord Kelvin, 
G.C.V.O. 
(Read January 18, 1897.) 
§ 1. Apparatus for realising the proposed method is represented 
in the accompanying diagram. Two Woulffe’s bottles, each having 
a vertical glass tube fitted air-tight into one of its necks, contain the 
liquids the difference of whose vapour pressures is to be measured. 
Second necks of the two bottles are connected by a bent metal (or 
glass) pipe, with a vertical branch provided with three (metal or glass) 
stopcocks, as indicated in the diagram. Each bottle has a third 
neck, projecting downwards through its bottom, stopped by a glass 
stopcock which can be opened for the purpose of introducing or 
withdrawing liquid. The upper ends of the glass tubes are also 
connected (by short india-rubber junctions or otherwise) with a 
bent metal pipe carrying a vertical branch for connection with a 
Toepler * mercury air-pump. This vertical branch is provided with 
a metal stopcock. The vertical branch of the pipe fitted into necks 
of the two bottles is also connected to the air-pump as indicated in 
the drawings. 
§ 2. To introduce the liquids, bring open vessels containing them 
into such positions below the bottles that the necks project down- 
wards into them. Close the glass stopcocks of these lower necks, 
open all the other six stopcocks, and produce a slight exhaustion 
by a few strokes of the air-pump. Then, opening the glass stop- 
cocks very slightly, allow the desired quantities of the liquids to 
«nter, and close them again. They will not be opened again 
unless there is occasion to remove the whole or some part of the 
* An ordinary mechanical air-pump would not serve the purpose, because 
its valves would not open properly to draw out the very small amount of air 
which must be removed to avoid vitiating the observations by any sensible 
amount of air-pressure added to the pressure of the vapour. 
