192 FLUIDITY AND PLASTICITY about its importance. One has only to compare the migration velocities of a series of ions with the fluidities of their chlorides in normal solution, as shown in curves I and II of Fig. 71 after Bredig, (1894), to see that there is a definite relationship between the two. In seeking an exact quantitative relationship we are met again by the awkward fact that water is associated and elec- trolytes in it perhaps always form hydrates, so that an apparently simple aqueous solution is not simple in fact. The study of molten salts, of liquid metals and alloys, and of non-aqueous solutions for this reason take on a particular importance, but aqueous solutions have naturally received the greatest attention. The method of investigation is usually to change the fluidity of the liquid by altering the temperature, concentration or pressure and to observe the corresponding change in conductivity. As early as in 1851 Wiedemann investigated the viscosity and conductivity of various salt solutions of varying concentration. Wiedemann calculated the value of the ratio m^/A, where m is the percentage of salt and A the conductivity. He found that the ratio varies within narrow limits for each salt, e.g., for copper sulfate the value varies from 22.8 to 24.2 when the concentration is increased from 31.17 to 187.02. Gour6 de Villemont^e in his monograph on Resistance Electri- que et Fluidite has used the results of Bouty and Bender to prove that the ratio nup/A. varies with the temperature in a manner which is the same for all salts. (Cf. Table L.) We have seen that over a small range of temperature so similarly A = A0(l + aO where a and /3 are arbitrary constants and