The facts described in the paper are commenced by one intended to illustrate future reasoning, by shewing what takes place when a very feeble electric current traverses a metallic circuit, interrupted in one part by a neutral solution, into which the two extremities of the wires forming the circuit are immersed. Two small copper wires were connected together by loops, and the two free ends joined to the ends of a galvanometer wire; the circuit was then cut in one place, and the extremities immersed in a solution of chloride of sodium. Then, if one of the loops be raised to a red heat by a spirit lamp, an electric current is produced, the heated loop furnishing negative electricity. Now if the ends plunged in the saline solution are terminated by platina or gold wires, no current of electricity is observed; with silver terminations, the current is very feeble; but with wires of zinc, lead, iron or tin, the current is very energetic. These remarkable effects, highly important in the phenomena hereafter to be considered, are no way connected with the conductibility of the metals; for lead and zinc, which are the worst conductors, are those which, with the copper, produce the most powerful effects. The current ceases altogether as soon as the lamp is removed.
As the zinc, copper, lead, and iron, belong to the class of oxidable metals, M. Becquerel concludes, from this experiment, that [p463] when very feeble electricities are generated in any point of a metallic circuit, interrupted by a saline solution, a current of electricity is formed or not, according as the two similar metallic terminations, which dip into the solution, belong to an oxidable or non-oxidable metal. If the saline solution be replaced by an acid, then a current will be obtained, though platina wires be used; because that kind of fluid does not interrupt the current.
With respect to the production of new compounds by electro-chemical powers, very much depends upon the strength of the power employed, and M. Becquerel only pretends, as yet, to indicate a new field of research, and not to point out the precise paths to be pursued. Two methods may be adopted. As an illustration, let a tube, from 4 to 8 hundredths of an inch in diameter, be bent into the form of the letter U, and place a plug of amianthus at the bend, to prevent the mixture of the fluids in the limbs: into one leg put a mixture of deutoxide of copper and solution of the sulphate of copper, the former will fall to the bottom; into the other put a saturated solution of common salt, and also an excess of the dry substance, then communicate the two fluids by a plate of copper. Very shortly the end plunged in the sulphate will be covered with metallic copper, and the acid set free will act upon the oxide of copper below and form more sulphate, so that a set of decompositions and recompositions will occur, and ultimately comparatively large crystals of copper will be obtained.
In the other branch of the tube, a portion of the salt will be decomposed, the muriatic acid will act upon the copper, which is oxidised in consequence of its positive state, and will probably produce an oxychloride, which will combine with the chloride of sodium, and then octoedral crystals will be formed on the plate of copper. The effects are produced either with or without access to air.
When the crystals are well dried and inclosed in a tube hermetically sealed, they suffer no change; but they are decomposed by water into chloride of sodium and submuriate of copper.
If the voltaic experiment be continued for one or two months, the crystals, from being colourless and limpid, become violet, and ultimately acquire an emerald green hue, still remaining transparent. If the chloride of sodium side be tested, it will be found that soda is evolved during the experiment. A piece of copper simply immersed in a solution of common salt, produces nothing more than a submuriate of copper, which precipitates.
With silver.—If a similar tube to that described have both limbs filled with a solution of salt, a platina wire introduced into one limb, a silver wire into the other, the extremities of the wire connected so as to form a voltaic circuit, and the whole left for some months, in about fifteen days crystals will be observed on the silver wire; these will gradually increase and assume a rhomboidal form. They have not yet been particularly examined, but [p464] are known to be unchanged by water: during a long experiment they change colour, becoming, first, violet, then blue.
Experiments similar to that with the copper, when repeated with the same solutions, &c., but the substitution of plates of lead and tin for the copper plates, produced crystalline double chlorides of these metals and sodium.
Muriate of ammonia being substituted for common salt in these experiments, another series of double compounds was obtained with copper, silver, lead, and zinc.
A double chloride of barium and lead was formed slowly in a similar way.