If a solution of gold be used, instead of that of silver, the platina becomes spee dily gilt. The experiment producing what is called the lead-tree cannot be accounted for in any other way : it con. sists in filling a bottle with a solution of acetate of lead, in the upper part off' which is suspended a piece of metallic zinc : in the course of a day or two, me tallic lead is observed in shining filaments, suspended from the piece of zinc. The same difficulty occurs in this as in the last experiment : the filaments of lead constantly grow from the ends at a dis tance of many inches from the zinc. In order to prove that this experiment is similar to the last, that is, that the lead is reduced by the hydrogen, take a tube, A B, fig. 3, at one end of which tie a piece of bladder so tight that the tube may hold water ; let a cork be inserted at A, through which the platina wire, P p, is passed ; the tube being set upright in the zinc cup, D, containing dilute muria tic acid, and a connection formed at P, the platina soon becomes covered with brilliant crystals of metallic lead : hence it would appear, that the platina had the power of reducing the lead into its me tallic state, or that some substance had been transmitted through the bladder adequate to that effect. If, instead of the acetate of lead, the tube be filled with dilute acid, upon the connection be ing formed at P, the platina becomes co vered with bubbles of hydrogen : need we, therefore, hesitate in concluding, that the lead owes its reduction to the hydrogen ? The method of whitening brass and copper, by boiling them with cream of tartar and tin, is a process of this kind ; the cream of tartar, and the metallic tin, answering the purpose of the zinc and acetate of lead in the last experiment : a portion of the tin in solution is reduced upon the copper or brass, rendering it white by the hydrogen which is produc ed during the galvanic contact of the copper or brass with the tin.
In all the experiments, the zinc wire is, during its contact with that of the pla tina, silver, &c. undergoing an increased oxydation, which is proportionate to the quantity of hydrogen evolved at the pla tina wire ; since the oxygen of that and hydrogen, both of which are derived from the water, are disposed of in the oxydation of the zinc. The hydrogen passes from the zinc to the opposite wire, with the greatest facility, through a di rect liquid communication, the shorter the better. It becomes much interrupted by having to turn sharp angles, or in pass ing through small apertures. It passes with more or less freedom through solid bodies, when moistened with water, but does not pass at all, except when moisture is present.
Having given an account of the effects resulting from a single galvanic combina tion, we will next give some account of the constructions of that compound appa ratus, termed Galvanic, or, more proper ly, the Voltaic battery.
The pile of Volta, of which we have already given a slight description, is at present so little used, that we shall direct our attention more particularly to the trough, as being more convenient for ex periments than the pile, and at the same time less liable to be out of order.
The wood of which the trough is form ed should be the oldest and hardest ma hogany, being less liable to warp than other kinds of wood. The sides of the
trough must be dove-tailed together, and the bottom ought to be grooved into the sides, and fitted in with turpentine ; per pendicular grooves must be made in the sides of the trough, for the reception of the plates, correspondent to which there must be grooves in the bottom. When the length of a trough is more than two feet, it becomes unwieldly ; it should not even be that length, when the size of the plates would render it too heavy to be handed about. The distance between the plates should be about three-eighths of an inch ; if they are nearer together, the acid employed is too soon exhausted, and, consequently, the power of the bat tery less lasting.
The plates should be of copper and zinc. Though silver is stronger than copper, it is not so in proportion to the price.
The zinc plates are best cut out of sheets of malleable zinc, as being cheap er, less liable to break, and may be used much thinner.
The copper may be employed so thin as six ounces to the square foot.
The plates of copper, being made a lit tle larger than the zinc, may be lapped over the edges of the latter, by which means they fit much closer to the zinc plate, without the labour of hammering the copper plates previously flat. The copper plates only require to be soldered to the upper edge of the zinc plate, since the other three edges are so secured with cement in the grooves as to preclude the necessity of soldering. The lapping over of the copper is sufficient to keep it close to the zinc plate till the plate is -fastened in the trough. Previously to in aerting the plates in the trough, the in side must be lined with a cement, form ed of resin and bees-wax, or, what is cheaper, of six parts of resin and one of lime and oil. The plates, being previous ly warmed, are to be pressed down into the grooves before the cement becomes quite cold. After the plates have been inserted, in such order that all the zinc surfaces shall face one way and the cop per the other, the cement must be more evenly adjusted with a hot iron which will reach to the bottom of the cells ; the trough being laid first on one side and then on the other for that purpose.
When the cementing process is finish ed, and the whole sufficiently cold, the trough must be dressed off, and varnished with copal varnish, where it can be had ; but in lieu of that with common spirit varnish. When the varnish is dry, it must be polished with rotten-stone and water.
In the above construction it is manifest that two of the surfaces are lost.by being laid and soldered together. About two years ago the writer of this article had conceived the possibility of making use of both the surfaces of the copper and zinc plates at the same time. According ly he cemented into a trough, in the groove made for the plates of metal, plates of glass. The metal plates were formed by soldering together a plate of each, of copper and zinc, and then bending them till the plates became parallel to each other, leaving a space between the two surfaces a little wider than the thickness of the glass plates.