274 SCIENCE PROGRESS 



that astronomical evidence can throw very little light on Dr. 

 Shaw's conclusion that the attraction depends upon the tem- 

 perature of the attracting mass. All astronomical measure- 

 ments of mass are made by measuring the attracting power ; 

 and it may be said that it is only an assumption to say that 

 this is proportional to the inertia, although experiments have 

 been made which tend strongly to support this assumption. 

 Dr. Shaw in Nature (vol. xcvii. p. 401, 191 6) indicates one 

 result which may throw some light upon the subject, viz., as 

 pointed out by Poincare, that the mass of Jupiter as derived 

 from the orbits of its satellites, as derived from its perturbations 

 of the large planets, and as derived from the perturbations of the 

 small planets has three different values. 



The final confirmation or otherwise of the result will no 

 doubt come from further experiments. Meanwhile, a whole 

 series of interesting questions are opened up ; e.g., Newton's 

 principle of the equality of action and reaction may be upset. 

 The results when finally established will help to decide between 

 some of the many theories of gravitation which have recently 

 been formulated. The difficulty in connection with such 

 theories has been that, as almost all gravitational experiments 

 have given negative results, it has been impossible to test them. 

 We await with interest further experiments in this direction. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc, 

 University, Liverpool. 



Electrolytic Dissociation. The Degree of Dissociation and the 

 Activity of an Ion. — As is well known, the relation <y=\ v /\ ao 

 which was first deduced by Arrhenius, and which gives the 

 degree of dissociation 7 of an electrolyte in terms of the 

 equivalent conductivity X„ at the dilution v and the equivalent 

 conductivity \ x at infinite dilution, is the fundamental equa- 

 tion in the theory of electrolytic dissociation. This very 

 simple relation contains the assumption that the mobilities 

 of the ions are independent of the concentration or dilution 

 of the system. Further investigation has shown, however, 

 that this assumption is not correct. The mobility varies with 

 the dilution. Since it seemed likely that the viscosity of the 

 system would affect the mobility, the original expression of 

 Arrhenius was modified in this sense, the resulting expression 



