368 SCIENCE PROGRESS 



is able to escape from the atom with the energy hv, the absorp- 

 tion being actually hv, plus the energy required to pull the 

 electron out of the atom ; or it implies that the incident light is 

 able to impart energy hv to a free electron in the metal. 



The point is not settled at the moment, but Millikan inclines 

 to the last alternative; 



PHYSICAL CHEMISTB.Y. By W. E. Garner, M.Sc, University 

 College, London, 



The Ionic Theory. — The theory of ionisation propounded by 

 Arrhenius in 1888 has withstood the criticism of the last thirty 

 years, and has survived the extensive experimental investiga- 

 tion carried out during this period. In spite of the incomplete- 

 ness of its application to moderately concentrated solutions of 

 strong electrolytes, no simple modification has yet been 

 devised which replaces the original theory in every-day use. 

 The conductivity of aqueous solutions can be satisfactorily 

 explained on no other assumption than that of the dissociation 

 of the electrolyte into ions. Uncertainty arises, however, as 



to the value of the conductivity ratio -j — for the determination 



of the degree of ionisation. In fact as G. N. Lewis points out, 

 it does not seem possible to ascribe any definite meaning to the 

 degree of dissociation. Since the law of mass action does not 

 apply to solutions of strong electrolytes, the existence of mole- 

 cules in the chemical sense is doubtful. 



The changes in the equivalent conductivity yl of a salt 

 in aqueous solution may be ascribed to variations either 

 in the mobility or in the number of the ions. Arrhenius 

 assumed that the mobility of the ions was independent of the 

 concentration, but grave doubt is thrown on the accuracy of 

 this assumption by Lewis, Maclnnes, and others. That rela- 

 tively large changes in the mobility of the ions do occur is 

 shown in the measurements of transport numbers of ions. 

 Maclnnes {J.A.C.S., 1921, 43, 121 7) points out that the anionic 

 transport number for hydrochloric acid varies from 0-167 to 

 0-156 between o-oi and i-oN, and that of lithium chloride from 

 0-668 to 0-694 for the same range. The transport number for 

 potassium chloride is, however, independent of the concentra- 

 tion. It seems probable that the mobilities of the ions of this 

 salt vary with concentration at the same rate, rather than that 

 their mobilities are constant. In any case, it appears that a 

 change in the mobility of the hydrogen ion of approximately 

 7 per cent, occurs between o-oi and i-oN. 



So far as conductivity and the mobility of the ions are 

 concerned, a simplication of the theory of dissociation would be 

 obtained if the assumption were made that strong electrolytes 



