Sbptembke 30, 1898.] 



SCIENCE. 



415 



tricity." When the current acted upon 

 caustic soda or potash, magnesium sulphate 

 or sodium sulphate, just as much hydrogen 

 and oxygen were evolved from them as 

 from the diluted sulphuric acid with which 

 they had been compared. And his final 

 declaration in regard to water ' that when 

 subjected to the influence of the electric 

 current a quantity of it is decomposed 

 exactly proportional to the quantity of 

 electricity which had passed.' Nor can I 

 refrain from a brief allusion to the elec- 

 trolysis of molten salts, also conducted by 

 Faraday. You all remember the experi- 

 ment he performed with lead chloride. 

 The weighed platinum-wire kathode was 

 introduced into the molten salt ; ' a button 

 of alloy could be observed gradually form- 

 ing and increasing in size.' In time the 

 experiment was interrupted, when it was 

 discovered that the 'positive electrode had 

 lost just as much lead as the negative had 

 gained * * * the equivalent number, by 

 comparison with the result in the volta- 

 electrometer, being 103.5.' Some one has 

 said, and I think we shall all echo the senti- 

 ment, that "the data communicated by 

 Faraday, as the result of his years of obser- 

 vation of the action of the electric current 

 upon chemical compounds, are among the 

 most important in the history of electro- 

 chemistry. They form the basis of all the 

 quantitative laws which have been de- 

 veloped in this special field. They merit 

 place side by side with the law of combin- 

 ing weights." 



Equally important, perhaps, both from a 

 theoretical and practical standpoint have 

 been the results recently won by the intro- 

 duction of the current into the field of pure 

 chemistry through the persistent efforts of 

 untiring investigators. Mark how in the 

 diffusion of solids in their solvents van t 

 'Hoff perceived a similarity to the expan- 

 sion of gases ; how he succeeded in meas- 

 uring that diffusive, expansive force bear- 



ing the name osmotic pressure ; then note^ 

 the astonishing consequences of this dis- 

 covery. The simple relation between gas^ 

 volume, attending pressure and temperature^ 

 had led Avogadro to promulgate his hy- 

 pothesis in regard to molecules — a thought 

 considered by all of us as a fundamental in 

 our science. This hypothesis van t' Hoflf 

 made bold to apply to solutions, and there- 

 resulted a theory for solutions analogous tO' 

 that previously wrought out for gases. To- 

 day molecular weight determinations are 

 made almost daily in our research labora- 

 tories as an indirect consequence of this de- 

 duction. Indeed, the laws governing os- 

 motic pressure, when applied in the most 

 varied forms, remained satisfactory, except 

 with solutions of salts, bases and acids. 

 Their osmotic pressure was far beyond what 

 could be expected from the theory of van 

 t 'Hoff. How was this to be explained ? 

 The answer came from Arrhenius (1887) : 

 the departure from the law is due to disso- 

 ciation, and most astonishing of all is that 

 the dissociation of bodies in their solvents 

 occurs only in the case of those which con- 

 duct the electric current, i. e., with elec- 

 trolytes. Compounds not dissociated in 

 aqueous solution — which adhere to the law 

 of osmotic pressure — do not conduct the 

 electric current. There remained but a step 

 to the assumption that the products of dis- 

 sociation, in aqueous solution, were iden- 

 tical with the substances termed ions, which 

 in electrolysis appear at the electrodes. A 

 logical query, consequent upon this declara- 

 tion, was : 



Are, for example, free potassium and free 

 chlorine present in an aqueous solution of 

 potassium chloride ? Let me read you Ost- 

 wald's well-known reply to this question : 



" What actually exists in the solution are 

 single potassium atoms with enormous elec- 

 trical charges. We do not know what these 

 charges are in reality, but this we know, 

 that the chemical properties of substanceB 



