Ordinarily one charge of liquid will serve for twenty-four hours working, but this, of course, is entirely determined by the space provided for it. It is sold at sevenpence a gallon, and each gallon is sufficient, we are informed, to drive a cell while it generates 800 ampere hours of current, or, taking the electromotive force at 1.8 volts, it represents (800 x 1.8) / 746 = 1.93 horse-power hours. The cost of the zinc is stated to be 35 per cent. of that of the fluid, although it is difficult to see how this can be, for one horse-power requires the consumption of 895.2 grammes of zinc per hour, or 1.96 lb., and this at 18_l_. per ton, would cost 1.93 pence per pound, or 3.8 pence per horse-power hour. This added to 3.6 pence for the fluid, would give a total of 7.4 pence per horse-power per hour, and assuming twenty lamps of ten candle power to be fed per horse-power, the cost would be about one-third of a penny per hour per lamp.
Mr Holmes admits his statement of the consumption of zinc does not agree with what might be theoretically expected but he bases it upon the result of his experiments in the Pullman train, which place the cost at one farthing per hour per light. At the same time he does not profess that the battery can compete in the matter of cost with mechanically generated currents on a large scale, but he offers it as a convenient means of obtaining the electric light in places where a steam engine or a gas engine is inadmissible, as in a private house, and where the cost of driving a dynamo machine is raised abnormally high by reason of a special attendant having to be paid to look after it.
But he has another scheme for the reduction of the cost, to which we have not yet alluded, and of which we can say but little, as the details are not at present available for publication. The battery gives off fumes which can be condensed into a nitrogenous substance, valuable, it is stated, as a manure, while the zinc salts in the spent liquid can be recovered and returned to useful purposes. How far this is practicable it is at present impossible to say, but at any rate the idea represents a step in the right direction, and if the electricians can follow the example of the gas manufacturers and obtain a revenue from the residuals of galvanic batteries, they will greatly improve their commercial position. There is nothing impossible in the idea, and neither is it altogether novel, although the way of carrying it out may be. In 1848, Staite, one of the early enthusiasts in electric lighting, patented a series of batteries from which he proposed to recover sulphate, nitrate, and chloride of zinc, but we never heard that he obtained any success.