The latter require to be remounted every 2 days, and do not give nearly inch a constant cu rrent, thus causing numerous troubles. These iron batteries have the remarkable property of being able to give with, out polarisation a duty of far higher force than corresponding non-metallic combinations; it seems that the surface of the iron must' be charged with occluded hydrogen, which is carried gradually as far as the oiide of copper, and thus assists the depolarising action

Lalande Chsperan battery.

Lalande-Chsperan battery.

It remains to design some practical reversible batteries, having a large and small duty. When an oxide of copper battery is exhausted, and a suitable current is made to traverse it, the oxygen at the positive pole is wholly absorbed by the reduced copper; but if the support of the depolarising material is of copper, a small quantity of the latter is dissolved, giving a blue colour to the liquid. The copper obtained by the reduction of the oxide, however, is not dissolved. By employing iron or cast-iron as a support this attacking and dissolution of the copper isa avoided, provided that the oxide used has been free at the outset from metallic copper. The great difficulty lies in the deposition of zinc. This metal is precipitated in a form which is not sufficiently coherent, particularly when it is required to have moderate thicknesses. This difficulty may be avoided either by precipitating the zinc on a very largo surface of brass or amalgamated copper (on a mass of copper shavings for instance), or by precipitating it on a horizontal surface of amalgamated brass, covered with an excess of mercury and furnished with depressions for this metal to lodge in. The under part of this support is farther covered with an insulator.

The zinc can then only arrange itself on the surface, and as- it immediately amalgamates, the deposit becomes coherent, and can, without loss, furnish a fresh amount of current by its dissolution.

Lalinde Chaperon battery.

Lalinde-Chaperon battery.

Moist - Imprimis, it should always be borne in mind that the circulation of the liquid is of fundamental importance in electro-chemical generators, so that all so-called " dry " (really moist) batteries necessarily fail in a most essential particular, and are even conceptually faulty: it is, therefore, clearly out of the question that a " dry " battery of a certain composition should ever be as efficient as a "wet" battery of the same composition, and the employment of a "dry *' battery can only be excused on the plea of convenience in handling - a plea that avails but in a few instances. , Even if the operations of gilding and plating could at all be conducted satisfactorily by the employment of " dry " cells, there would be no reason whatever for preferring them to ordinary cells; as a matter of fact, however, a " dry " cell is comparatively useless for such a purpose. It will undoubtedly ring a bell, and if large enough and moist enough it may do so for a long time; it will also plate, but no one, except perhaps the "dry battery" manufacturer, has found an advantage in having -plating cells dry.

The current of a dry cell is, moreover, very inconstant, and the internal resistance is high. (H. G. L.) Being simple and convenient, they naturally recommend themselves to the inexperienced amateur who will not think for himself. Such batteries only act as long as their packing is moist and chemical action possible. That action does not last long, because "the new product which results from the oxidation of the zinc being practically a solid, cannot fall to the bottom by its higher specific gravity, as it does in the liquid cell; it remains where it is formed, and shields the zinc against the further action of the packings inventors pretend that the waste products are absorbed and new moisture regenerated. That is impossible in a dry cell. The zinc cylinders of these cells offering a large surface, the chemical action must go on for some short time before all the moisture is neutralised; so long a current will flow. Still, the non-removal of the waste product must speedily prove fatal to every dry cell.

Regeneration can be effected in different ways: by passing a current through them and making them a sort of accumulator, or by diluting a solution. But who will undertake that trouble, when the ordinary Leclanche, which costs not half as much, does all the work for more than a year without any regeneration? True it is, that competition has brought the price ot the latter cells so low that it scarcely pays to make them well, and that they often fail on that account. Respectable houses will always sell a proper article, because they know how to, and it costs no more to make. There are occasions when dry cells are very suitable. When you go out testing or bell-hanging, it is more convenient to carry a small dry cell than a liquid one, and even so with the medical electrician. In such cases lasting time is not of much importance.

(A. C.)

I made up a dry battery which was in use 6 months for bell ringing; and although of small size and very simple, it is equal to two Leclanche cells. I cast a slab of plaster of Paris with a little oxide of zinc with it, mixing it with water as thin as I could for it to set well, and when thoroughly set I dried it in an oven; when dry I soaked it in a strong solution of chloride of zinc; and mixing a spoonful more of plaster with the same solution, I spread it over one side of the slab, and pressed a zinc plate on it before it was set. Then I did the same on the other side, and put on a plate of lithanode, of course leaving one end of each projecting for binding screws. Afterwards I rolled paraffined paper round the whole, and tied it up tight with string. Its simplicity, cheapness, and portability leave nothing to be desired as far as bell work l2 goes, and I think it would be useful for continuous-current work. (A. L.)