Zinc can only combine with sulphuric acid by liberating an equivalent amount (chemically speaking) of hydrogen - thus Zn + H2S04 = ZnS04 + H2 , and just as zinc gives up energy and generates an electromotive force, so hydrogen absorbs energy, but in lesser amount, when set free from the combination, generating at the same time a " back " or negative electromotive force of about 1*46 volt; consequently the resultant E.M.F. of the cell is only the difference of these two actions, or about •9 volt (2.31-1.46), and the rest of the energy passes away in the escaping gas. If, however, some material is provided for which the hydrogen has a strong affinity, and with which it can re-combine, once more yielding up the whole or a portion of its potential energy the E.M.F. of the cell will be increased in proportion, and this is one of the chief objects accomplished by the depolarising fluid. In the Daniel cell, with a solution of copper sulphate for the "depolariser," the hydrogen displaced by the zinc in its turn displaces the copper - thus, CuS04 + H2 = H,S04. Cu, the metal being deposited on the negative pole; but the actual gain of E.M.F. is not very large, because the greater portion of the energy released by the hydrogen is absorbed by the liberated copper, which has nearly as strong an affinity for S04 as the hydrogen itself, and exerts a back E.M.F. of 1.25 volt.

The resultant E.M.F. of the cells is now made up as follows: - 2 .36 - 1.46 + 1.46-1-25, the two middle figures representing the liberation and recombination of the hydrogen to form sulphuric acid, of course, neutralise each other and cancel out, leaving 2.36 - 1 25 = 1 • 1 volt approximately, which means that more than half of the energy of the dissolved zinc has merely been transferred to the deposited copper.

The action of the depolariser in the Grove, Bunsen, and bichromate cells, including their various modifications, is, from our present point of view, very much more effective; but as the chemical changes are more complicated, and their exact nature uncertain, they cannot be followed with the same precision as those in a Daniel cell. In every case, however, the liberated hydrogen abstracts oxygen from the depolariser to form water, nitric acid being broken up into oxide of nitrogen, and chromic acid being reduced in the same way to oxide of chromium, which combine with the sulphuric acid present to form sulphate of chromium; but very little energy is absorbed in effecting these decompositions, and most of the energy of the hydrogen is returned to the circuit, which accounts for the high E.M.F. of the cells.

If a zinc plate is placed in dilute sulphuric acid with the positive plate of an ordinary lead accumulator - that is, a lead plate covered with lead peroxide (PbO2) - we have a combination which gives an E.M.F. of 2.36 volts, showing that it is possible for this cell to utilise the whole of the energy electrically. As with the other depolarisers* hydrogen abstracts oxygen from the peroxide, reducing it to PbO, which combines with the sulphuric acid to form lead sulphate -PbO2 + H2=PbO + H20 and PbO + H2S04=PbS04+H2O.

Many kinds of zinc-consuming bat teries have long been used for working electric telegraphs, bells, telephones, and other similar apparatus; but none of them has ever been successfully applied to the production of electricity commercially and on a large scale for the purposes of electric-lighting, motor-driving, etc. The great difficulty in the way is that of cost. As long as small currents only are required, the expense of the material consumed in generating them is a relatively small one, and does not need much consideration, especially when, as is often the case, there are no other means of accomplishing the object in view; but as a method of producing electrical energy in quantity, batteries have to compete with other and cheaper ways of gaining the same end. For instance, zinc is a far more expensive fuel than coal, although the potential energy of the latter has first to be converted into mechanical energy in a steam-engine, and then into electricity by a dynamo, while the former effects the conversion at one step.

The amount of zinc consumed in producing an electrical horse-power (746 watts) for one hour = 2/E lb. (E represents the working E.M.F. of one cell) supposing there were no loss by local action. IŁ the battery were generating the full E.M.F. that is theoretically possible, namely, 2.36 volts per cell, this would give 2/2.36 =.85lb., and, although it is improbable that the terminal E.M.F. of any battery would, when at work, much exceed 2 volts per cell, as some loss due to the internal resistance must take place, we will assume that this minimum figure can be attained; then, zinc costing 3d. per lb., an electrical horse-power hour would cost 2 1/2d. for zinc alone.

Now, the combustion of lib. of coal generates at least five times as much energy as llb. of zinc. On the other hand, a steam-engine will only convert 10 per cent., or about one-tenth of this into mechanical work, consequently, the energy to be obtained from llb. of coal burned in a steam-engine is 5/10 = .5, or one-half of that to be obtained from 1lb. of zinc consumed in a battery. In other words, 21b. of coal will be required to produce one horse-power hour, which agrees with the results actually obtained from good engines working under favourable conditions.

Beckoning coal at 16s. per ton, 21b. costs 1/6d., which will be the cost of the material used to obtain one mechanical horse-power hour in this way, and if this is transformed into electricity by means of a dynamo, the cost of an electrical horse-power hour may be put at a 1/4d., which will allow ample margin to cover the losses of this second conversion.

The relative cost, therefore, of energy produced from zinc and from coal is as 2 1/2d. to 1/4d., or as 10 to 1, and this without making any allowance for the expense of the exciting fluid and thedepola-riser, which would cost at least as much again as the zinc itself. (E. J. Wade.)