SO,,II The discharge, therefore, effects a deposition of copper at the copper, and the formation of sulphionide of hydrogen at the porous cell, and of sulphionide of zinc at the zinc rod. Instead of hydrogen in its nascent state being deposited at the copper, we htive copper in the same condition; but the galvanic polarization caused by the hitter is very much inferior to that resulting from the former, and hence the supe rior electromotive force of Daniell's cell. The porous cel•eeps the sulphate of zinc from reaching the copper, and thus obviates another source of diminished force in the one-fluid battery. The sulphate of zinc once formed, is itself subjected to the decom posing action of the pile, and zinc is deposited on the copperplate, thus tending to give a zinc-zinc instead of a copper-zinc The constancy of Daniell's battery is not unlimited, for the sulphate of zinc which results from the action, being a bad conduc tor of electricity, enfeebles the current. In the Daniell cell used for teleglitphs, the containing vessel is of glass, and no sulphuric acid is added to the water round the zinc, as, in the course of the action, sufficient acid conies through from the sulphate cell. • Grove's Battery consists of platinum-zinc couples. Fig. 3 shows an excellent arrange ment of a cell of it„ The outer cell of glass, g, is filled with dilute sulphuric acid (1 part of acid to 8 of water), in which a cylindric plate of zinc, 2, is immersed. Inside the zinc is it porous cell, (1, containing coucentra ed nitric acid and the platinum plate, p, which is bent into the form of an S (fig. 4), to increase its surface, Grove's couple is very much superior in powtr to any of the preceding, though it is inferior in constancy '''to Daniell's.
When the poles are joined, sulphate of zinc is formed in the outer cells and hyponitric acid vapors are given off by the nitric acid. vapors are injurious to the health when breathed for any time, the porous cell is closed with a stopper of wood, to prevent or lessen their escape, the connection between the exterior and the platinum plate being made by a strip of metal passing through the wood.
The chemical action of Grove's couple may be shown as before, taking nitric anhS-dride to be the oxide of nitric peroxide Before dis charge, the molecules stand thus, begin ning with the platinum: Pt,I\12040,. N2040, H2SO4, Zn : and after it, Pt,N204, The nitric peroxide (N904) discharged at the platinum plate is absorbed by the nitric acid, in which it is soluble, so that the plate is left free. The cells of a Grove's battery are connected with the platinum of the one to the zinc of the other.
Bunsen's cell has the same chemical action as Grove's, the plati num being replaced by carbon. There are two forms of the cell—the one invented and erirployed by prof. Bunsen, and generally adopted in Germany; and the modification introduced by Archerau, generally found in England and France. The Bunsen cell, properly so called, has a carbon cylinder immersed in nitric acid, and the porous cell zinc containing the zinc and sulphuric acid placed within it. In a battery of these cells,
the of one cell is connected with the carbon of the next. In the second form, the same arrangement is adopted as in Grove's cell. The latter form of the.Bunsen cell. in con sequence of the preponderance of the positive surface, gives the greater quantity of electricity. For the manufacture of the Bunsen carbons, see CARBON FOR ELECTRIC PURPOSES. Bunsen's battery, in point of cheapness, is preferable to Grove's, where the platinum forms an expensive item, but is inferior to it in point of compactness. In these couples, the platinum and carbon may be replaced by iron, which is nearly as electronegative as either in concentrated nitric acid. The el'ectro-motive forces of the various cells expressed in volts, arc Grove's, 1.92; Bunsen's, 1.88: Daniell's, 1.079; Since's. 0.47; and Wollaston's, 0.39. The resistance in Daniell's cell is much greater than in Grove's or Bunsen's.
Different Forms of the Davy Cell: This cell is used exten sively on French telegraphic lines with the best results. It is much smaller in size than the smallest Daniell cell, the containing glass vessel being little more than 3 inches in height. It has been 'found that 38 eleMepts-do as much as 011couples of Daniell, and keep in action twice as long, requiring no replenishing for half a year. The chemical action is much the same as dna of Daniell's. The cell is thus charged; powdered mer curous sulphate. is treated with water, a basic insoluble salt is formed, and falls to the bottom of the vessel, but a small amount of a mercurous salt is left dissolved in the water. The clear liquid is decanted off and a paste is left. The carbon is placed in the porous cell, and the vacant space is filled with the paste thus got. The glass ves sel-outside containing the zinc is charged with pure water and a little of the decanted liquid.. The connections are made by straps of lead instead of copper, to avoid the action of the mercurous salt on the latter metal. The small quantity of mercurous salt in the zinc cell has the excellent effect of keeping the zinc constantly amalgamated.— Bunsen's Bichromate Cell: Bunsen sometimes charges his cells with a liquid consisting of from 100 to 150 parts, by weight, of water, to which 12 parts of bichromate of potash and 25 parts of sulphuric acid have been added. No porous cell is needed with this charge. the carbon being kept from touching the zinc by hempen cords or the like.— The Bichromate Cell, as generally used in this country, is thus made up. The liquid with the carbon in the porous cell is in the proportion of 10 oz. of water to 1 oz. of bichro mate of potash and 5 oz. of sulphuric acid. In Bunsen's bichromate charge, zincic sul phate is formed and potassium chrome alum. last-named bichromate cell, zincie chloride is formed in the zinc cell, and potassic and sodic chrome alum in the porous cell.