It was in the prosecution of these experiments, while he was examining the effect of different conducting substances placed between the plates, that M. De Luc was led to the discovery of the curious instrument, called the Electric co lumn ; a pile consisting of a number of discs of zinc and gilt paper, placed alternately upon each other, and included in a glass tube. This has already been described under the article ELEcTureiTy, and as it must he regarded as a strictly electrical apparatus, we shall not enter into any de tails respecting its effects or its mode of action. See Ni cholson's Journal, xx vi. 39.
While Sir I I. Davy and De Luc were thus enlarging our knowledge of the powers of galvanism as a chemical agent, and of the means by which its wonderful effects are accomplished, Mr Children was advantageously employing himself in improving the apparatus. He formed a battery, constructed upon a principle originally suggested by Vol ta, according to which the plates are not cemented toge ther, but are connected only at the top by a metallic con ductor, and are then immersed in the cells of a trough. (Plate CCLXIII. Fig. 5.) He employed 20 pair of plates, of four feet by two, making in all a surface of 92,160 square inches. The fluid that he used was a diluted mixture of nitric and sulphuric acids, the whole quantity being no less than 120 gallons. The effect of these large plates was to fuse entirely, in about 20 seconds, IS inches of platina wire, of one-thiitieth of an inch in diameter, and to render three feet of the same wire red hot. Charcoal was burned with intense brilliancy. It seemed not a little remarkable, con sidering the powerful effect on platina wires, that the ac tion of this battery on iron wires was comparatively trifling, Of iron wire, 1-70th of an inch in diameter, it barely fused ten inches, and had not power to ignite three feet. It had not the power of decomposing barytes and other similar substances ; it did not affect Bennet's electrometer ; and it seemed scarcely able to produce a perceptible shock.
Mr Children next formed a battery of 200 pairs of plates of two inches square, affording a surface of 3200 inches. With this the alkalies and alkaline earths were readily de composed, and a considerable divergence was produced in the gold leaves of the electrometer. From this compari son of the effects of the two batteries, we are led to the conclusion which has been already referred to, that the intensity of the electricity is increased with the number, and the quantity of it with the extent, of the metallic plates. Upon this principle, we may explain why the platina wire was acted upon more readily than the iron wire, the more perfect conducting quality of the former presenting no ob stacle to the passage of the electricity through it ; while the tendency of the iron to oxidation required a greater in tensity of the fluid to effect its transmission through the wire. In this paper the author states, that he has remov
ed one of the objections that have been urged against the identity of the galvanic and the common electricity, that the former has no striking distance ; by employing a proper apparatus, he ascertained that the galvanic spat k was ca pable of passing through a certain space between the ex tremities of two platina wires.
Mr Children's general conclusion is, that " the absolute effect of a voltaic apparatus is in the compound ratio of the number and size of the plates ; the intensity of the electri city being as the former, the quantity given out as the lat ter; consequently, regard must be had in its construction, to the purposes for which it is designed. For experiments on perfect conductors, very large plates are to be prefer red, a small number of which will probably be sufficient ; but where the resistence of imperfect conductors is to be overcome, the combination must be great, but the size of the plates must be small. But if quantity and intensity be both required, then a great number of large plates will be necessary. For general purposes, four inches square will be found to be the most convenient size." See _Phil. Trans. for 1809, p. 32.
Mr Children has since constructed a still larger and more powerful battery, consisting of 20 pairs Of copper and zinc plates, each plate being six feet by two feet eight inches. It ignited six feet of thick platina wire, and melt ed platina with great facility ; it also melted iridium and osmium. At the suggestion of Dr Wollaston, a singular fact was ascertained, that a greater length of thick platina wire was ignited, than of platina wire of a much smaller size. See Thomson's Annals, ii. 147.
We have given some account of a paper of Erman's, in which he endeavours to show, that certain bodies arc what he calls Unipolar, that is, that they are conductors of one kind of electricity only. Mr Brande conceived, that the facts brought forward by Erman, might admit of a better explanation upon a different principle, viz. that some che mical bodies, being naturally positive, and others naturally negative, they would be attracted to the surface of the pile in a contrary state to their own, the positive to the negative, and the negative to the positive surface.