432 M. BECQUEREL ON CHEMICAL DECOMPOSITION AND 



lution of hyposulphite of potash, obtained by decomposing in the air 

 some protosulphuret of potassium. One end of a wire or plate of pure 

 silver is then plunged into each of them. The reaction of the two so- 

 lutions on each other, and that of the hyposulphite on the plate of silver, 

 produce electrical effects, in consequence of which, the plate immersed 

 in the nitrate becomes the negative pole of a voltaic apparatus. The 

 nitrate of silver is slowly decomposed, the plate immersed iu it is co- 

 vered with silver in a metallic state, while the oxygen and the nitric 

 acid pass into the other branch, where they concur in the formation of 

 a double hyposulphite of silver and potash, which crystallizes in beau- 

 tiful prisms ; but as the oxygen and the acid continue to arrive, they 

 react on this combination and the hyposulphite of potash : there are 

 then formed sulphate and nitrate of potash, and sulphuret of silver, 

 which remains unchanged so long as there is not a quantity of nitric acid 

 sufficient to act on it. The formation of the sulphate and the nitrate 

 of potash is easily explained ; but the case is far otherwise with the 

 formation of the sulphuret. Let us consider the circumstances by 

 which it is accompanied. In proportion as the liquid evaporates in the 

 positive branch, we see at the bottom of the tube and above the 

 clay some pretty octahedral crystals of sulphuret of silver formed 

 on the plate of silver. These crystals resemble, in apj^earance, those of 

 the same substance that are found in silver mines. Like them, they 

 extend themselves lightly under the hammer ; their colour is a leaden 

 gray, and their exterior surface is dim. The resemblance, indeed, is so 

 close that the artificial cannot be distinguished in any respect from the 

 natural crystals. 



Why is it that, in consequence of the reaction of the oxygen and the 

 nitric acid on tlie hyposulphite, we obtain a sulphuret of silver instead 

 of a hyposulphite, a sulphite, or even a sulphate ? This question can- 

 not be answered but by supposing that the positive pole acts on the 

 oxide of silver and the hyposulphurous acid so as to disoxidize them ; 

 when the silver and the sulphur, being in their nascent states, obey 

 their mutual affinities. As these effects are produced slowly, there 

 is nothing to oppose the regular grouping of the particles of sulphu- 

 ret of silver. Nothing of a similar kind is obtained with a solution of 

 sulphuret of potassium. In this case the results of the experiment are, 

 sulphate of potash and sulphate of silver. This is very probably to be 

 ascribed to the influence of the proportions and the energy of the 

 action. 



Sulphuret of Copper. — In order to apply the foregoing principles to 

 the formation of other sulphurets, and first to that of sulphuret of 

 copper, let us substitute for the solution of nitrate of silver a solution 

 of nitrate of copper, and for the plate of silver a plate of copper : there 

 is quickly formed in that side of the tul)e which contains the hyposul- 

 piiitc of potash, a double hyposulphite oi' copper and potassium, which 



