482 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 in 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 appearance, 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 the hyposulphite, we cbtain 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 tube which contains the hyposul- 
phite of potash, a double hyposulphite of copper and potassium, which 
