(1) Determinations have been made, at 18° C, of:—(a) the specific electrical conductivity, (b) the percentage composition by weight, of a series of mixtures made by saturating, with sodium chloride, solutions of hydrochloric acid containing from 16 to 27 per cent, of hydrogen chloride.

(2) Within this range of concentration the salt-saturated acid has lower specific conductivity than the aqueous solution of hydrogen chloride alone, from which it may be supposed to be derived. Of the salt-saturated acid mixtures, that one has maximum conductivity which could be prepared by adding salt to hydrochloric acid containing 2·9 per cent, of hydrogen chloride. Of solutions of hydrogen chloride alone, in water, that containing 19·1 per cent, hydrogen chloride has maximum conductivity.

(3) The critical concentration of hydrogen chloride at which hydrochloric acid is able to affect the colour of cobalt chloride is changed, in the same sense as the concentration of hydrogen chloride corresponding to maximum conductivity is changed, by saturating the solution with salt.

(4) A new form of conductivity cell is described. It is specially suitable for solutions which are saturated with dissolved solid, or contain a volatile solvent.