Sept. 29, 1887] 



NATURE 



52f 



either electrode, only at the last moment attempting a brief and 

 unavailing struggle, when the electrode suddenly looms foreign 

 and forbidding across a molecular distance of io~ * centimetres. 



One mode of regarding the facts is to say that across this 

 molecular range of lO" ^ the electrical forces are competent to 

 tear atoms asunder. The E.M.F. of a volt or so can be shown 

 by calculation to be able to do this, so that the difference between 

 an electrolyte and a dielectric may be typified diagrammatically 

 thus : — 



Electrolyte. 



Dielectric. 



The two vertical lines are electrodes, the slant or broken line represents the 

 slope of potential in the two cases respectively. 



Prof. Schuster has now discovered one way in which di- 

 electrics shade off into electrolytes ; for he finds that in the 

 neighbourhood of an electric discharge rarefied gases are able to 

 conduct as electrolytically as liquids themselves. This discovery 

 that the atoms of gas possess atomic charge as well as those of 

 liquids, if confirmed by further research, is one of considerable 

 interest. 



But why do we assert the horizontality of the line of slope in 

 the fluid ? Why do physicists feel constrained to assert that no 

 internal static electric stress is possible in the interior of a mass 

 of fluid ? The question is but the paraphrase of another — Why 

 do we believe liquids to obey quite accurately Ohm's law for 

 very minute forces? On this head we have direct experi- 

 mental evidence by Prof. FitzGerald and Mr. Trouton, and 

 less direct but equally conclusive evidence from von Ilelmholtz. 

 Whether the evidence is perfect and thorough is doubtless a 

 debatable point, but this much is not debatable : it is out of 

 the question to assert that liquids obey Ohm's law and at the 

 same time to assert the existence of a finite electrostatic stress in 

 the interior of a fluid. In other words, however chemists are 

 able to explain the fact of unresisting atomic processions through 

 the liquid — whether by actual procession of individuals or by 

 continual directed interchange — they will be rigorously driven to 

 some form of such doctrine as soon as they accept the evidence 

 for the accuracy of Ohm's law in electrolytic conduction. ^ 



We all know that this doctrine of non-resistance is in some 

 shape or another the old Williamson-Clausius hypothesis, which 

 was based on then newly-known facts concerning dissociation. 



It would appear, however, that some chemists demur to the 

 existence of a constant average of dissociation among the mole- 

 cules of a liquid ; and it behoves us of Section A to receive 

 their scruples with great respect, being, we may suppo ,e, based 

 upon intimate familiarity with all manner of circumstances and 

 reactions of which we physicists are only superficially cognizant. 



But there are ways of picturing all that is necessary to free 

 atomic interchange without postulating actual and constant disso- 

 ciation. A potential dissociation will be granted, sufficient for 

 all purposes, provided chemists admit the probability of a 

 frequent interchange of atoms among the molecules of an 

 electrolyte going on always before any E.M.F. has been 

 applied. 



Concerning this or other mode in which electrolytic conduc- 

 tion takes place, we may congratulate ourselves on the presence 

 here of Prof. Quincke and Prof. Wiedemann, and we hope to 

 hear something from them. The experiments of Dr. Gladstone, 



' This sentence must be modified in the final report, because of some 

 interesting observations of a controversial character made by Prof. FitzGerald 

 at the meeting concerning it. 



and also some unpublished ones of Prof. J. J. Thomson, com- 

 municated to the Committee in a letter, will probably be found 

 to have a bearing on this point. 



The question whether there is any radical distinction to be 

 drawn between ordinary compounds and so-called molecular 

 compounds appears to be an open one. Various physical facts 

 lead one to suppose that whereas the ordinary forces of chemical 

 affinity are strictly electrical there may be other non-electrical 

 forces as well, and that such compounds as are held together by 

 these latter forces are intractable to electrical influence. It is 

 difficult for physicists to understand certain facts without the 

 hypothesis of some non-electrical forces between atoms ; but on 

 such a subject as this chemists are likely to have in their hands 

 evidence which, if at all decided and distinct, would be entitled 

 to very great weight. 



The subject of the partition of the current among different 

 electrolytes when mixed together, and the question of the part 

 the solvent plays in the conduction seem scarcely suitable for 

 discussion at the present stage, because they only require a few 

 rigorous experiments on lines already laid down to settle them. 

 But I may just say that whereas at a former meeting I thought I 

 had obtained experimental evidence that the water conducted 

 some fourth part of the current in certain solutions, I have since 

 found thai, using purer substances, and taking extreme care to 

 avoid loss of weight by spray, which source of loss is very subtle, 

 this evidence puts on another complexion ; and at the present 

 time I am disposed to coincide more cordially with the orthodox 

 view, that water conducts almost as little when forming part of 

 a solution as when existing alone. Further experimental evidence 

 is still being obtained, however, and perhaps Mr. Shaw has 

 something to communicate on this head. 



Among several communications received by the Committee from 

 non-British philosophers is an exceedingly suggestive one by Prof. 

 Willard Gibbs, which raises a very interesting point. 



It is perfectly well known that in 1851 our present chairman. 

 Sir William Thomson, reasoning from some experiments of 

 joule, taught us how to calculate the E.M.F. of a cell from 

 thermo-chemical data, 



E = 2(Je9) ; 



or — volts. 



46000 



Strictly speaking he hedged with regard to reversible heat 

 effects in a way equivalent to the complete equation — 



E = 2(j€e)-2(jn) (1) 



Where n, is the heat developed at junction i per unit quantity 

 of electricity conveyed across it, IIj the same at the second 

 junction, and so on. 



But the value of n, in any given case, is extremely difficult to 

 measure, especially at metal-liquid and liquid-liquid junctions. 

 Bouty has attempted it with but small success. 



Fortunately Helmholtz has thought of applying the second law 

 of thermodynamics to the subject, and shown that it was only 

 necessary to know the rate at which the E.M.F. of a cell varied 

 with temperature in order to know the sum of the IT. For, 

 quite analogous to Prof. James Thomson's freezing-point 

 relation — 



is the following E.M.F. relation — 



^E5Q = J^J^5H, 



2n = 



SH Ta'E 



5Q~J^T 



Putting the two equations together we get- 



JTe/ 



(2) 



(3) 



which we may say is certainly true. 



But now Prof. Willard Gibbs suggests a novel mode of apply- 

 ing the second law or doctrine of entropy. 



He takes into account the temperature of dissociation, or 

 temperature at which the reaction could reversibly take place"; 

 and, calling this T^, he writes the E. M. F. at any actual tem- 

 perature T thus — 



T — T 

 E = jOe iil— i (4) 



