THE ELECTROLYTIC DISSOCIATION THEORY 383 



which it correlates, and by the impetus which it has given to 

 chemical investigation. One or two of the observations for which 

 it offers a satisfactory interpretation may here be referred to. 



(1) The properties of dilute solutions are generally additive 

 in character, and can usually be represented as the sum of two 

 factors, one pertaining to the positive, the other to the negative 

 part of the solute molecule. This is true of the electrical 

 conductivity (Kohlrausch, 1876), the specific gravity (Valson, 

 1 871), the refractive index (Bender, 1890), the internal friction 

 (Reyher, 1888), the magnetic rotation (Jahn, 1891), etc. The 

 rotatory power of an optically active acid is, in dilute solution, 

 independent of the base with which it is combined, and the same 

 is true of the light-absorbing power of solutions containing 

 coloured ions (Ostwald, 1892). All these observations at once 

 become intelligible on the theory of the independent existence 

 of ions. 



(2) The phenomena of hydrolysis are readily accounted for 

 on the assumption that water itself is slightly ionised. The 

 amount of ionisation has been determined by four distinct 

 methods, and the agreement is satisfactory. Further, the tem- 

 perature coefficient of the conductivity of the purest obtainable 

 water, as determined by Kohlrausch, agrees with that calculated , 

 from Van 't Hoff's equation representing the influence of tem- 

 perature on the equilibrium. 1 



(3) The, at first sight, surprising fact that on neutralisation 

 of equivalent amounts of different strong acids by strong bases 

 in dilute solution the same amount of heat is set free, also finds 

 a ready explanation on the theory. Since ionisation in dilute 

 solution is practically complete, the process of neutralisation of 

 potassium hydroxide by hydrochloric acid, for example, is repre- 

 sented by the equation, K'+ OH'+ H'+ Cl'= K/ + CI' + H 2 + x 

 calories — in other words, the only reaction in this and other 

 similar cases is the combination of H' and OH' ions to form 

 water, so that the amount of heat evolved in each case is neces- 

 sarily equal for equivalent quantities. 



No other theory so far proposed gives an adequate repre- 

 sentation of the observations enumerated above. More or less 

 plausible attempts 2 have been made to account for the abnormal 



1 Van 't Hoff, Lectures on Physical Chemistry, English edition, i. p. 157. 

 * Poynting, Phil. Mag. 1896, 42, 298 ; Armstrong, Encyc. Brit. 10th edition, 

 vol. 26, p. 741 ; Kahlenberg,yisw/j. physical Chem. 1906, 10, 141. 



