380 EECENT PEOGEESS IN PHYSICS. 



tion of whicli, to avoid interruptions, we have deferred, viz : tlie 

 deposits upon the diaphragms and the accumulation of the fluid at the 

 negative pole, 



Faraday had already found by placing the positive pole in a con- 

 Pig. 157. centrated solution of sulphate of magnesia 



^ ^ and the negative in water floating upon that 

 solution, (the experiment being most con- 

 veniently made in the manner indicated by 

 ■^gure 157,) that by the passage of a very 

 powerful current magnesia was deposited 

 at the surface of contact of the two fluids. In 

 the water oxygen passes toward the + po^e 

 and in the solution of sulphate of magnesia 

 magnesium toward the — pole ; and when 

 they meet each other at the dividing surface of the two fluids they 

 combine to form magnesia, which is precipitated. Similar observa- 

 tions are noticed by Daniell, in the above mentioned memoir. 



He filled a glass cylinder, closed at the bottom by means of a piece of 

 bladder, with a dilute solution of caustic potassa, and suspended it so 

 that it just dipped below the surface of a concentrated neutral solution 

 of sulphate of copper in a wider glass vessel. He then placed the 

 negative pole of a battery of twenty cells in the solution of potassa 

 and the positive in that of the sulphate of copper. At the negative 

 pole hydrogen was evolved, at the positive oxygen, while the bladder, 

 from which also some gas arose, was copiously coated v^^ith metallic 

 copper interspersed with oxide of copper and blue hydrated oxide. 

 This action is thus explained. The sulphate of copper is decomposed 

 into S O4 and Cu.; the ibrmer passes to the positive pole, the latter to the 

 bladder, where it is arrested, and gives up its charge to thQ hydrogen 

 of the other electrolyte, the solution of potassa, v/hich then passes on 

 to the negative pole and is evolved. On the other hand, the oxygen of 

 the solution of potassa on its way towards the positive pole is arrested 

 by the bladder, transfers its charge to the S O4 passing in the same 

 direction, and combines with the copper it meets at the bladder. 

 But this action is too rapid for this combination to be complete, a 

 part of the copper is precipitated in the metallic state upon the blad- 

 der, while a part of the oxygen escapes as gas. 



In fact, the metallic copper diminishes in quantity as the force of 

 current decreases. With feebler currents the bladder was covered with 

 a thick coating of oxide of copper, in which only a few spangles of 

 metallic copper were visible. 



From nitrate of silver much metallic silver was deposited upon the 

 membrane, mixed with oxide of silver. Gas, too, was disengaged 

 from the diaphragm. The whole of the oxygen was not evolved at 

 the positive pole, but a part of it combined by secondary action with 

 the oxide of silver in the solution to form a peroxide. 



Mfra.te of lead gave similar results. Froto-nilraie of mercury 

 exhibited the most remarkable phenomenon. Not only were globules 

 of mercury formed upon the membrane, but a shower of small globules 

 fell from the diaphragm throughout the experiment. 



§173. Accumulation of fluid at the negative 'pole. — Porret first ob- 



