354 BEPORT — 1887. 



Comparison between the Views of Dr. Arrhenius and Professor Armstrong 

 on Electrolysis. Reply to Professor Lodge's Criticisms. By Henry 

 E. Armstrong, F.B.S. 



Professor Lodge in bis summary very clearly points out the ditference in the 

 views advocated by Arrhenius and myself, and emphasises the chief points which 

 it is of importance to discuss. He unreservedly condemns my hypothesis, on the 

 ground that any E.M.F., however small, is sufficient to produce sensible electrolysis ; 

 and he asserts this to be a proof of the correctness of the orthodox view that the 

 E.M.F. has nothing to do but to give direction to the already separated atoms. 

 I have before expressed my doubt of the force of this argument ; but 1 may add 

 that it appears to me hopeless to attempt the experimental disproof of such a "state- 

 ment, our ability to prepare pure substances being out of all proportion small as 

 compared with our power of detecting electric currents: indeed, it woidd be pre- 

 sumption to attempt to contend with an engine of such surpassing delicacy as ' a 

 galvanometer sensitive enough to show a current which could only decompose a 

 milligram of water in a century.' 



It is apparently desirable that I should more fully state, from a chemist's point 

 of view, the chief reasons which cause me to hesitate in accepting the ' atomic 

 dissociation hypothesis,' and which have led me to suggest an alternative ' mole- 

 cular hypothesis,' viz., that in the case of ' composite electrolytes,' at all events, 

 electrolysis is the outcome of the combined action of the E.M.F. and of some efiect 

 which the one set of molecules exerts upon the other set while both are under the 

 influence of the E.M.F. I care little at present what the eflect is, the important 

 question to settle being whether electrolysis is primarily an aflfair of atoms or of 

 molecules. 



1. In explanation of the fact that neither hydrogen chloride (HCl) nor water 

 (HjO) is an electrolyte, although a solution of the one in the other conducts 

 readily, it has been sometimes assumed that the dissociated atoms of H and CI 

 are shielded and prevented from recombiniug by the intervention of the neutral 

 molecules of the solvent, opportunity being thus given for the E.M.F. to act and 

 give du-ection to the atoms. But a sinr/k substance such as fused silver iodide, 

 for example, conducts readily, and is electrolysed. How are we to explain this ? 

 I imagine that the orthodox view also in this case requires us to assume that 

 there are, normally present in the iodide, dissociated iodide and silver atoms ; just 

 as it is assumed that there are atoms of H and CI in hydrogen chloride, or of H 

 and in water. Why, then, does electrolysis take place in the one case, but not 

 in the other ? The conductivity of such a substance as silver iodide is far too con- 

 siderable to be explained by the assumption that it contains impurity, which, 

 judging from the behaviour of aqueous solutions, could not possibly be present in 

 sufficient amount to account for the readiness with which the electrolysis takes 

 place. Nor can we, with any degree of probability, suppose that either 'hydrogen 

 chloride or water in the pure state consists wholly of Arrhenius's complex inactive 

 molecules ; that silver iodide is at all events rich in simple active molecules ; and 

 that such simple active molecules are produced from hydrogen chloride only on its 

 dilution with water. Nor do I conceive that it helps us to assume that a compound 

 of hydrogen chloride with water is formed ; it does not appear to me to be probable 

 that an aggregate of the form (HCI)„ • (OH,),, would be more susceptible of elec- 

 trolysis than the component simple molecules, and that these would be more likely 

 to suffer dissociation when associated than when free. Unwieldy aggregates, such 

 as Professor Lodge refers to, break up, I imagine, if at all, not because some of the 

 outlying atoms are not held with full vigour, nor because the atoms of the one 

 constituent molecule are by collision of the aggregates brought close to those of 

 the other, but because the atoms of the molecules which form the aggregate are 

 brought into intra-molecular relationship ; i.e., the break up is not the result of the 

 collision, but of the opportunity thus given for re-pairing to take place ' and 

 possibly new simple molecules, not atoms, always result. 



' This view is in entire agreement with one I expressed in the communication 

 referred to (page 353) ; indeed, it may be considered a paraphrase of it.— O. L. 



