598 



NA TURE 



\_Apnl 21, I J 



now so large that our sense of proportion demands that atten- 

 tion should no longer be directed almost exclusively to the study 

 of carbon compounds. 



There is one other problem to which I would direct attention 

 — the study of liquid diffusion. No interpretation of the re- 

 markable results obtained by Graham has yet been given, and 

 they appear in many cases to be quite at variance with the 

 results of chemical inquiries. But there is a striking parallelism 

 to be observed between Graham's results and those obtained on 

 determining the electrical conductivity of solutions. -Applying 

 the view which I hold regarding the electrolysis of composite 

 electrolytes to liquid diffusion, it appears to me not improbable 

 that diffusion may be to a large extent the outcome not so much 

 of the proper motion of the molecules of the dissolved substance 

 as of a propulsive action exercised by the molecules of the 

 solvent. The molecules in a mass of water we know may be 

 assumed to be moving in every direction, and this being the case 

 they would tend to carry other molecules along with them : the 

 extent to which this action would take place would, however, 

 largely depend on the attraction which exists between the mole- 

 cules of water and those of the dissolved substance. From this 

 point of view it appears of considerable importance to extend the 

 study of liquid diffusion to dilute solutions. It may be added 

 that this hypothesis would probal ly account for the behaviour of 

 colloids, .as these are known to be chemically neutral substances ; 

 in fact, they are compounds almost destitute of residual affinity. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



King's College, London. — Prof. W. Grylls Adams, F.R.S., 

 will deliver a course of lectures on Electro-Magnetism, Mag- 

 neto-Electricity, the Testing of Motors and Dynamos, Electric 

 Lighting, and Transmission of Power, during the present Term. 



A course of practical work in Electrical Testing and Measure- 

 ment with especial reference to Electrical Engineering will also 

 be carried on under his direction in the Wheatstone Laboratory. 



The lectures will be given once a week — on Mondays, at 

 2 p.m., — and the Wheatstone Laboratory is open daily from 

 I to 4, except on Saturdays. 



SOCIETIES AND ACADEMIES 



London 



Royal Society, March 31. — "Note on the Development of 

 Voltaic Electricity by Atmospheric Oxidation." By C. R. 

 Alder Wright, D. Sc. , F. R.S., Lecturer on Chemistry and 

 Physics, and C.Thompson, F.C.S., Demonstrator of Chemistry, 

 in St. Mary's Hospital Medical School. 



Whilst investigating processes for the manufacture of cupram- 

 monium hydroxide (now used commercially on a considerable 

 scale) we noticed that if the air supply be greatly in deficiency 

 relatively to the bulk of the copper, under certain conditions the 

 solution is but little coloured, containing copper dissolved prin- 

 cipally as cuprous, and not as cupric, oxide. This might, per- 

 haps, be anticipated a priori, inasmuch as it is well known that 

 blue cupric solution in ammonia, when digested with metallic 

 copper in the absence of air, takes up a second equivalent of 

 copper, becoming colourless cuprous solution ; but further ex- 

 periments seem to indicate that the production of cuprous oxide 

 under the oxidising influence of a limited supply of air is the 

 primary action, and not merely a secondary result. 



When a sheet of copper is kept out of direct contact with air 

 by being immersed in ammonia solution, oxidation of the metal 

 is gradually effected by virtue of the dissolving of oxygen from 

 the air at the surface of the fluid, and diffusion of the oxygen 

 solution to the vicinity of the copper. This action is an ex- 

 tremely slow one if the copper be covered by some depth of fluid, 

 and if the setting up of convection currents through heating or 

 evaporation be prevented by keeping the vessel perfectly at rest 

 and at an equable temper.ature, and well closed to prevent 

 escape of ammonia ; but if these precautions be neglected it 

 goes on much more rapidly, and the liquid comparatively soon 

 becomes blue ; it can, however, be also materially accelerated 

 by arranging horizontally on the surface of the fluid a plate of 

 platinum or other electrically conducting material not chemically 

 acted upon by the fluid, and connecting this by me.ans of a wire, 

 &c., with the copper plate. The upper conductor, ox aeration 



plate as it may be conveniently termed, being simultaneously in 

 contact with the atmosphere and fluid, attracts to its surface a 

 film or aura of condensed gases, the oxygen of which becomes 

 gradually transferred to the copper, a voltaic current circulating 

 through the fluid and connecting wire. Cuprous, and not 

 cupric, oxide thus results, dissolved in the ammonia solution in 

 contact with the copper plate, the mechanism of the reaction 

 being conveniently represented by the scheme — 



j" Copper plate Cuj | OH., | OH.^ | O .Aeration plate, 



\ Copper plate | CujO | H,0 | H.p | .Aeration plate, 

 water being represented as the electrolyte for simplicity's sake. 

 The air film on the .leration plate being constantly renewed by 

 absorption from the atmosphere, the process goes on continuously 

 as long as the two plates are connected together by the wire. 

 This wire may be lengthened at will so as to make the current 

 which passes through it whilst the action goes on relatively 

 stronger or weaker according to the amount of resistance intro- 

 duced into the circuit ; and by including a galvanometer or silver 

 voltameter in the circuit the ordinary phenomena due to the 

 passage of currents are readily recognisable. 



The m.aximum E.]\LF. thus capable of development varies 

 considerably with the strength of the ammoniacal solution, 

 being the less the weaker the fluid ; addition of common salt or 

 of sal ammoniac to the liquid notably increases the E.M.F. and 

 diminishes the internal resi-tance of the cell. Spongy platinum 

 in a thin layer as the aeration plate gives higher values than thin 

 platinum foil ; the highest numbers thus obtained, using pretty 

 concentrated ammoniacal brine, fell but little short of o'8 volt ; 

 or somewhat less than the E. M.F. corresponding with the heat of 

 formation of cuprous o.xide,' since, according to Julius Thomsen, 

 Cu„,0 = 4o,Sio - about o'88 volt. 



It is obvious that this copper atmospheric oxidation cell has a 

 close connexion with the "air-battery" described in 1873 by 

 Gladstone and Tribe (Roy. Soc. Proc, vol. xxi. p. 247) in 

 which what is virtually an " aeration plate," consisting of a tray 

 full of crystals of silver is used, opposed to a copper plate im- 

 mersed in a solution of copper nitrate. Cuprous oxide is 

 formed in both cases, in virtue of the indirect combination 

 brought about between the oxygen of the air and the copper : 

 but there is this great difference between the two (apart from the 

 cuprous oxide being deposited as such in Gladstone and Tribe's 

 arrangement, and being kept in solution in ours), that in the 

 one the cuprous oxide is formed at the surface of the copper plate 

 itself, and in tlie other at the surface of the aeration plate. This 

 essential difference is embodied in the above depicted scheme as 

 compared with the following one which represents the action in 

 Gladstone and Tribe's cell : — 



Silver + 0l*^"'N°^>n^"<N0»'2 

 ^"^^'^ + ^|Cir(N03)„|Cu(N03)., 



Cu I ^''PP"''- 



! c-i I r> Cu I (NO,).,Cu I (NO,).,Cu I „ 



1 S''^^'- I Ocu I (NO^j.Cu I (No'ixu I C°PPe>-. 



One result of this difterence is that the surface of the aeration 

 plate in the ammonia cell is kept constantly the same, where.as 

 in the nitrate cell it is continually changing its character through 

 deposition of solid cuprous oxide on the silver: inconsequence 

 of this deposition, whilst the E.M.F. of the ammonia cell, 

 cateris paribus, is constant, that of the nitrate cell is continually 

 varying. Gladstone and Tribe, moreover, only obtained an 

 E.M.F. of ^"tj to W of a Daniell, or about o'io4 to OT43 volt, 

 even under the most favourable conditions, viz. when the cell 

 was connected with an electrometer ; whilst four or five times 

 this amount is indicated by the cells examined by us. 



Following up the ideas suggested by the above observations, 

 we aire making a number of experiments with a variety of 

 analogous combinations, in which atmospheric oxidation con- 

 stitutes the essential chemical .iction taking place ; by varying 

 the nature of the aeration plates, the metals dissolved, and the 

 liquids employed (as also by substituting other gases, e.g. 

 chlorine, for air), a large number of combinations are obviously 

 obtainable. Some of those which we have so far examined 

 present points of considerable interest, the oxidising action 

 exerted under favourable conditions being strongly marked : so 

 much so that certain metals, e.g. niercuiy and silver, not ordin- 



^ The actual chemical change going on in the cell is the synthesis of 

 cuproso-ammonium hydroxide, so that the (unknown) heat of solution of 

 cuprous oxide in ammonia should be added to this to obtain the total heat 

 development. 



