140 Royal Society ;— 



action to prepare copper nitrate absolutely free from dissolved oxygen. 

 An ordinary solution of the salt mixed with some silver nitrate was 

 placed in a narrow cylinder, with a long piece of copper-foil arranged 

 somewhat spirally, so as to retain the deposited silver on its surface, 

 and allowed to rest for twenty-four hours. The solution thus obtained 

 was exposed to the action of the conjoined copper and silver plates ; 

 but even after some hours there was no dimming of the lustre of the 

 silver plate, except at the air-line, which was sharply defined. The 

 same solution, shaken for some time in the air, produced a yellowish 

 deposit on the white metal in three minutes. 



The colour and general appearance of this crust, together w^ith its 

 formation only where oxygen can be absorbed, showed that it was 

 not metalHc copper, but the suboxide. This was further proved by 

 the action of dilate sulphuric acid, which resolves it at once into 

 red metallic copper and copper sulphate. There is also another 

 curious reaction, which can only be properly observed under a micro- 

 scope. When treated with a solution of silver nitrate, this cupreous 

 deposit does not give the ordinary crystals of the white metal; in 

 fact it is only slowly acted upon ; but presently there shoot forth thin 

 threads of silver, which run through the liquid, often twistiiig at 

 sharp angles, while the yellowish crystals change to black. This 

 also was found to be a property of the suboxide of copper. 



This deposition of oxide on the silver is accompanied by a corre- 

 sponding solution of copper from the other plate. Thus, in an ex- 

 periment made with nitrate-of-copper solution that had been exposed 

 to air, and which was allowed to continue for four days, there was 

 found : — 



Gain of silver plate 0*016 grm. 

 Loss of copper plate O'Ola grm. 



The copper necessary for the production of O'OIG grm. of suboxide 

 would be a little above 0"014 grm. 



The wire connecting the two plates in this experiment is capable 

 of deflecting a galvanometer. The current takes place through the 

 fluid from copper to silver — that is, in the same direction as if the 

 copper had been dissolved by an acid and hydrogen evolved on the 

 silver plate. 



If the two plates have their sides parallel, the suboxide is deposited 

 not raerel}^ on that side of the silver plate which faces the copper, 

 but after about a minute on the other side also, showdng that in 

 this, as in other cases, the lines of force curve round. 



It became interesting to consider wdiat started this electric current. 

 The original observations convinced us that it was not due to the action 

 of oxygen on the copper ; but, to make the matter more certain, 

 bright copper and silver plates in conjunction were immersed, the 

 copper in a pure, f. e, deoxygenized, solution of nitrate of copper, the 

 silver in an oxygenized solution : the two liquids communicated 

 through the diaphragm of a divided cell. In half an hour the silver 

 plate was covered with a reddish film, while not a trace of tarnish 



