The primary object of these experiments was to obtain a series of accurate determinations of the V. P. of aqueous NaCl solutions at 18° C., which might be used as a scale of reference for the determination of the V. P. of other salt solutions at the same temperature by the isopiestic method. The ionisation laws of salt solutions sufficiently dilute are broadly known, but for more concentrated solutions the derivation of ionisation from conductivity data is complicated by changes in the composition of the water itself and in the amounts of water combined with and travelling with the ions. An attempt to resolve some of these difficulties was made in a former paper by the use of V. P. data derived by extrapolation from Tamman's observations at temperatures from 40° C. to 100° C. Most of the reliable conductivity determinations are for a temperatures of 18° C. necessarily introduced inaccuracies. To carry the matter further more accurate determinations at 18° were necessary. By the method of isopiestic solutions when an accurate scale of reference for one salt is obtained, it is easy to obtain equally accurate data for other solutions. In the illustration described in that paper four pots containing solutions of KCL, NaCl, LiCl, and KNO 3 were put into a desiccator which was evacuated and immersed in a thermostat kept at 18°C. until equilibrium was established by evaporation from one solution to obtain a series of isopiestic solutions of equal V. P. A portion of such s series is set out in Table VI later. The reference series used was the LiCl V. P. series, the values of which were determined by extrapolation from Tamman's data.

Vapour pressure and density of sodium chloride solutions. (1923). Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 103(722), 429–443.