3o8 



NA TURE 



[July 27, 189; 



liave recently been discovered between light and electricity 

 which lead us to believe that the latter is most closely con- 

 nected with the luminiferous ether." 



" Clerk-Maxwell showed that the ratio of two electrical con- 

 stants which are capable of being determined by laboratory 

 experiments, and which are of such a nature that that ratio 

 expresses a velocity, agrees with remarkable accuracy with the 

 known velocity of light. This formed the starting-point of the 

 electro-magnetic theory of light which is so closely associated 

 with the name of Maxwell." 



" According to this idea, light may be looked on as the propa- 

 gation of an electro-magnetic disturbance, whatever the appro- 

 priate idea of such a thing may actually be. The theory has 

 quite recently received remarkable confirmation by the investi- 

 gations of Hertz, who has shown that what are incontestably 

 electro-magnetic disturbances, and are investigated by purely 

 electrical means, exhibit some of the fundamental phenomena of 

 light, such, for example, as interference and polarisation. It 

 appears that these electro-magnetic waves are strictly of a 

 similar nature to the waves of light, though there is an enor- 

 mous difference in the scale of wave-lengths, which in the case 

 of light range about the ifnJjrffth part of an inch, while the 

 electro-magnetic waves which have been investigated by purely 

 electrical methods range from a few inches to many yards." 



" I have ventured to bring this interesting subject before you 

 in the course of the address which I have just delivered. I 

 have not attempted to lay before you the evidence on which 

 scientific men rely for the truth of the conclusions which I have 

 mentioned as well established. That would have required, not 

 merely an evening address, but a whole course of lectures. 

 Neither have I made any allusion to possible bearings of the 

 scientific conclusions on questions relating to religious beliefs. 

 .■\nything of that kind I leave to your own minds ; my object 

 has been simply to present to you very briefly the conclusions 

 of science in that limited branch which I have selected, distin- 

 guishing as impartially as I could what is well established from 

 what is debatable or even merely conjectural. " 



THE NA TURE OF DEPOLARISERS> 



"\X^HEN an electric current is passed between plates of platinum 

 through a solution of sulphuric acid, the hydrogen and 

 oxygen are partly retained at the surfaces — and apparently also 

 within the plates — and under these conditions are capable of 

 interacting, as in the well-known Grove gas battery : so that in 

 so far as the "gases" thus circumstanced are concerned the 

 change may be expressed by a reversible equation. This 

 reversal constitutes the well-known phenomenon termed polarisa- 

 tion by physicists. 



Reversal owing to the retention of hydrogen in circuit is pro- 

 moted to different extents by different metals — hence apparently 

 the varying electromotive forces of single fluid cells containing 

 different negative plates ; and when the pressure is sufficient to 

 retain the whole of the hydrogen at the plate, it becomes total 

 — hence it is, for example, that zinc does not dissolve in 

 sulphuric acid under great pressure. 



Various substances known generally as depolarisers are used 

 to prevent the accumuhation of products of electrolysis and the 

 consequent reversal of the action — such as copper sulphate in 

 the case of the Daniell cell and "nitric acid" in the case of the 

 Grove and Bunsen cells ; but whereas the action of copper sul- 

 phate is easy to understand, that of " nitric acid " offers many 

 difficulties. As the heat of dissolution of copper in dilute sul- 

 phuric acid is a negative value (about 12,000 units), the dis- 

 placement of copper by hydrogen — i.e. the heat of dissolution 

 of hydrogen in copper sulphate — is a positive value, so that not 

 only does the presence of the copper sulphate prevent the accu- 

 mulation of hydrogen, but in removing hydrogen it also serves 

 to increase the electromotive force of the cell from about 

 37/46ths to about So/46lhs of a volt. The principle underlying 

 this is extensible even to cases in which one part of the cumula- 

 tive effect of the cycle of change is a negative value. Thus, 

 although copper has a negative heat of dissolution, it will readily 

 dissolve in dilute sulphuric acid if it be used in place of zinc in 

 a Grove cell, the negative heat of dissolution of copper being 

 more than compensated for by the positive heat of dissolution of 

 hydrogen in "nitric acid"; and it is well known that copper 

 •dissolves in many weak acids in presence of oxygen. It is 

 ' Reprinted from the Proceedings of the Chemical Society, No. 125. 



NO. 1 239, VOL. 48] 



easy to understand how oxygen acts in such cases, but the facts 

 show that the effect produced by "nitric acid " is not so readily 

 interpreted, and their consideration raises important questions of 

 general application. 



Russell has shown that when "nitric acid" is freed from 

 nitrous compounds it does not dissolve silver, but that action 

 sets in when a trace of nitric oxide is introduced, and continues 

 with increasing rapidity as the quantity of the nitrous compound 

 — a necessary product of the action — increases ; Veley's later 

 experiments have shown that the same is true of copper, with- 

 out, however, affording any further explanation of the pheno- 

 mena. .Although it is not to be expected that such irelals 

 would dissolve in nitric acid even when coupled with a rela- 

 tively electronegative conductor, as they have negative heats of 

 dissolution, yet if the acid also acted as depolariser a cycle 

 might be formed in which sufiicient energy would be developed 

 to condition change : it therefore follows that in such cases 

 nitric acid does not act as the depolariser in accordance with 

 the equation: 2Ag + 2NO3H -f ONOjH = 2AgN03 -f H,0 

 -t- NO,H, and that in point of fact the nitrous compound is the 

 depolariser, although the nitric acid is the actual solvent of the 

 metal, the hydrogen of the acid being virtually directed dis- 

 placed by the metal with the assistance, however, of the current 

 energy derived from its own oxidation by the nitrous compound. 



But what interpretation is to be given of the behaviour of 

 more active metals, such as zinc, magnesium, &c. , which have 

 positive heats of dissolution, and therefore are capable (f dis- 

 solving in the pure dilute acid if coupled with a relatively nega- 

 tive conductor ; does nitric acid in their case directly act as a 

 depolariser? If it be capable of thus acting, such metals even 

 when uncoupled should dissolve in the pure diluted acid. It is 

 noteworthy that when such metals are dissolved in nitric acid 

 hydrogen is sometimes evolved. It has been suggested that this 

 hydrogen is derived from the interaction of the metal and water, 

 but I cannot now regard this as a probable explanation ; its 

 production serves rather to suggest a deficiency of the depolar- 

 ising agent, which cannot well occur if nitric acid be the de- 

 polariser. Indeed, if nitric acid be regarded as directly active, 

 it is remarkable that in presence of the large excess of the acid 

 which is always present any hydrogen should escape ; and also 

 that the reduction should extend so far as it often does, ard not 

 extend merely to the formation of nitrous acid. If, however, 

 the acid be incapable of directly acting as a depolariser, ind a 

 nitrous compound be the initially active depolarising agent, it is 

 no longer surprising that owing to the nitrous compound .'.uffer- 

 ing further reduction it should be deficient in parts of the cir- 

 cuit, and that consequently hydrogen should escape. Why the 

 reduction should extend so much further when metals having 

 positive heats of dissolution are used, however, still require* 

 elucidation. 



In the case of sulphuric acid, whatever metal be dissolved ia 

 the (/zVwM/ acid, no reduction takes place ; and it is only when the 

 concentrated and more or less heated acid is used that sulphurous 

 oxide and other reduction products are obtained. It appears not 

 improbable that reduction only takes place under conditions under 

 which the presence of sulphuric o.\ide is possible, i e. that de- 

 polarisation is effected by sulphuric oxide ani never by sulphuric 

 acid, although this latter may be regarded as the actual solvent 

 of the metal. There is at present no evidence forthcoming to 

 show that nitric acid can dissociate into the anhydride and 

 water, and even if such a change took place in concentrated 

 solutions, there is no reason to assume that it can also take 

 place in tlilute solutions, and that this is the explanation of the 

 difference between nitric and sulphuric acids. It is well known, 

 however, that nitric acid is resolved with extreme facility into 

 nitrogen dioxide, water and oxygen, and that it is exces- 

 sively sensitive to the action of nitric oxide — a trace of nitnc 

 oxide would therefore exercise a fermentative action and condi- 

 tion, the formation, it may be, of nitrous acid, or — as there 

 is no evidence compelling us to suppose that the compound 

 represented by the formula HNOn exists — it may be of nitro- 

 gen dioxide. In this latter case, solutions of nitric acid would 

 resemble concentrated sulphuric acid in containing a reducible 

 oxide, and it may be that their depolarising action is initially 

 exerted through such an oxide alone. 



To arrive at a clear conception of the function of acids in 

 dissolving metals, and of the nature of depolarising agents, 

 it would therefore appear to be necessary to take into account 

 many circumstances to which hitherto but little attention has 

 been paid. 5 Henry E. Armstrong. 



