1872.] 



Action of Oxygen on Copper Nitrate. 



291 



the solution gradually filled the jar again by the absorption of the gas. In 

 the same way the oxygen was absorbed from air, or from its mixtures with 

 hydrogen or carbonic anhydride. 



This action was further studied by employing plates of the two metals 

 instead of copper covered with silver crystals. When the two plates, con- 

 nected by a wire, were partially immersed in an ordinary aqueous solution 

 of copper nitrate, it was found that a slight yellowish deposit made its ap- 

 pearance speedily all over the silver plate, and went on increasing for a day 

 or two, while at the air-line there was a thicker deposit, which gradually 

 grew and extended itself a little below the surface. This deposit changed 

 from yellowish to red, and under the microscope presented a distinctly 

 crystalline appearance. 



Thinking that this slight crust all over the silver plate was due to air 

 dissolved in the solution itself, we took advantage of the reaction to pre- 

 pare 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, pro- 

 duced a yellowish deposit on the white metal in three minutes. 



The colour and general appearance of this crust, together with its forma- 

 tion only where oxygen can be absorbed, showed that it was not metallic 

 copper, but the suboxide. This was further proved by the action of dilute 

 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 microscope. 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 

 twisting 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 corresponding 

 solution of copper from the other plate. Thus, in an experiment 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*0 1G grm. 

 Loss of copper plate 0*0 15 grm. 



The copper necessary for the production of 0*016 grm. of suboxide would 

 be a little above 0'014grm. 



The wire connecting the two plates in this experiment is capable of de- 

 flecting a galvanometer. The current takes place through the fluid from 



