§ 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 downwards 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 stopcocks very slightly, allow the desired quantities of the liquids to enter, and close them again.