Fig. 5.

The silver sheet, before being placed in position, is platinised; that is to say, its surface is covered (by electro-deposition) with a coating of platinum, in the form of a fine black powder. This presents innumerable points of escape for the hydrogen gas; and for this reason this battery falls off much less rapidly than the plain zinc and smooth copper form. A modification of Smee's battery which, owing to the large negative surface presented, is very advantageous, is Walker's graphite cell. In this we have a plate of zinc between two plates of gas-carbon ("scurf"), or graphite. The surface of this body is naturally much rougher than metal sheets; and this roughness of surface is further assisted by

coating the surface with platinum, as in the case of the Smee. The chief objection to the use of graphite is its porosity, which causes it to suck up the acid fluid in which the plates stand, and this, of course, corrodes the brass connections, or binding screws.

Other mechanical means of removing the hydrogen have been suggested, such as brushing the surface of the plate, keeping the liquid in a state of agitation by boiling or siphoning; but the only really efficient practical means with which we are at present acquainted are chemical means. Thus, if we can have present at the negative plate some substance which is greedy of hydrogen, and which shall absorb it or combine with it, we shall evidently have solved the problem. This was first effected by Professor Daniell; and the battery

known by his name still retains its position as one of the simplest and best of the "constant" forms of battery. The term "constant," as applied to batteries, does not mean that the battery is a constancy, and will run for ever, but simply that so long as there is in the battery any fuel (zinc, acid, etc.), the electrical output of that battery will be constant. The Daniell cell consists essentially in a rod or plate of zinc immersed in dilute sulphuric acid, and separated from the copper or collecting plate by a porous earthen pot or cell. Around the porous cell, and in contact with the copper plate, is placed a solution of sulphate of copper, which is maintained saturate by keeping crystals of sulphate of copper (blue stone, blue vitriol) in the solution. Sulphate of copper is a compound built up of copper Cu, and of sulphur oxide SO4. When the dilute sulphuric acid acts on the zinc plate or rod ([§ 18]), sulphate of zinc is formed, which dissolves in the water, and hydrogen is given off:—

Zn+H2SO4=ZnSO4+H2.
Zincandsulphuric acidproducezinc sulphateandfree hydrogen.

Now this free hydrogen, by a series of molecular interchanges, is carried along until it passes through the porous cell, and finds itself in contact with the solution of copper sulphate. Here, as the hydrogen has a greater affinity for, or is more greedy of, the sulphur oxide, SO4, than the copper is, it turns the latter out, takes its place, setting the copper free, and forming, with the sulphur oxide, sulphuric acid. The liberated copper goes, and adheres to the copper plate, and, far from detracting from

its efficacy, as the liberated hydrogen would have done, actually increases its efficiency, as it is deposited in a roughened form, which presents a large surface for the collection of the electricity. The interchange which takes place when the free hydrogen meets the sulphate of copper (outside the porous cells) is shown in the following equation:—

H2+CuSO4=H2SO4+Cu.
Free hydrogenandcopper sulphateproducesulphuric acidandfree copper.