CRYSTALLIZATION PRODUCED BY VOLTAIC ACTION. 437 



ing a greater affinity towards this base than the nitric acid has. The 

 latter acid being liberated proceeded to the positive pole, whei'e it com- 

 bined itself with the oxide of lead, formed, in ' a great measure, at the 

 expense of the oxygen of copper. The result was a nitrate of lead, 

 Mhich crystallized in proportion as it had saturated the alcohol. With 

 a more powerful voltaic action, the sulphate of potash would have been 

 decomposed. This example shows how valuable the feeble electric 

 currents are in the production of combinations. In the case under 

 consideration, the nitrate of potash was decomposed by the concur- 

 rence of the electric forces and the chemical affinities. 



Let us now apply the foregoing principle to the decomposition of the 

 sulpho-carbonate of potash, and the formation of some compounds. 



1st Experiment. — The sulpho-carbonate of potash, whose solution 

 when not very much concentrated is gradually decomposed in the air, 

 is jieculiarly disposed to admit of changes being produced in the state 

 of combination of its molecules by the action of feeble forces. The 

 following is the mode in which we operate on this substance : we take 

 two glass jars, and pour into the one a solution of sulphate of copper, 

 and into the other an alcoholic solution of sulpho-carbonate of potash. 

 The communication between the two liquids is then established, on 

 the one hand by means of a bent glass tube, filled with potter's clay 

 moistened with a solution of nitrate of potash ; and on the other by 

 means of a metallic arc formed of two plates, one copper and the 

 other lead, the plate of copper being immersed in the sulphate, and 

 the plate of lead in the sulpho-carbonate. According to the nature of 

 the electric effects produced in the various chemical reactions, the lead 

 is found to be the positive pole of a pile whose intensity is sufficient to 

 decompose the sulphate ; the copper is reduced ; the oxygen and the 

 sulphuric acid are carried towards the lead ; the acid, in its passage, 

 decomposes the nitrate of potash, as in the preceding experiment, so 

 that the oxygen and the nitric acid enter alone into the sulpho-carbo- 

 nate. As soon as they have penetrated it, they begin to react on its 

 constituent parts, and this reaction continues so long as the force of the 

 current is superior to the affinities of the several bodies present : the 

 transfer of the molecules is continued as far as the positive plate, 

 where the last reaction takes place. The following products are suc- 

 cessively formed : neutral carbonate of potash which crj'stallizes on the 

 sides of the vessel, sulphate of potash, a sulpho-carbonate of lead and 

 of potash in acicular crystals, carbonate and sulphate of lead in 

 needles ; and, in fine, a portion of sulpluir proceeding from the decompo- 

 sition of the sulphuret of carbon and the sulphuret of potash, is depo- 

 sited on the plate of lead, which is the positive pole, and there crystal- 

 lized in octahedrons with a rhomboidal base. These octahedrons some- 

 times attain a millimetre in length when the expei'iment has been con- 

 tinued for a month. 



