42 MORRIS LOEB 



salts of nickel, copper, cobalt, etc., upon dilution and heating, 

 effects which Arrhenius ascribes to a difference in color be- 

 tween the free ion and the compound. Armstrong and 

 Pickering present the "residual affinity" theory with much 

 vigor. The atomic valences are not assumed to be whole 

 numbers, so that it rarely happens that the positive and 

 negative valences in a compound just balance. Consequently, 

 a small residual affinity remains to each molecule, by virtue 

 of which larger aggregations are formed, molecular com- 

 pounds, either among the molecules of the same sort or with 

 the molecules of the solvent. Armstrong's valences represent 

 the old Berzelian negative and positive electricities, which 

 cause their respective atoms to tend to opposite electrodes 

 during the passage of the current, the atoms being set free 

 by the complexes "straining" at each other as they pass 

 each other. The idea of unequal electrical charges appears 

 incompatible with Faraday's law that the current produces 

 the same effect in all electrolytes. Pickering bases his opposi- 

 tion upon more modern ideas, separating affinity from elec- 

 tric charges, and sees in the phenomena of electrolysis and 

 dilution effects of the dissociation and formation of various 

 hydrates. The basis for this lies in the observations by Men- 

 deleeff, Pickering, and Crompton, that various properties of 

 acids and salts, such as conductivity, density, the heat of 

 solution, when plotted as ordinates, with the percentage 

 composition of the solution as abscissas, do not present reg- 

 ular curves. A study of their first or second differentials 

 would appear to reveal the presence of points of abrupt 

 change, and these points are supposed to represent definite 

 chemical compounds. It does not appear that Arrhenius has 

 entirely invalidated the proofs of the existence of such com- 

 pounds, and there is no doubt that they present a very 

 awkward obstacle to his theory, which he has still to sur- 



