or:—
| Sulphuric Acid | and | Zinc, | equal | Zinc Sulphate | and | Hydrogen Gas. |
| H2SO4 | + | Zn | = | ZnSO4 | + | H2 |
Or we may put this statement into a general form, covering all cases in which zinc is acted on by a compound body containing hydrogen, representing the other or electro-negative portion of the compound by X:—
Zn + H2X = ZnX + H2
the final result being in every case the corrosion and solution of the zinc, and the extrication of the hydrogen gas displaced.
§ 19. We learn from the preceding statements that no electricity can be manifested in a battery or cell (as such a combination of zinc acid and metal is called) without consumption of zinc. On the contrary, we may safely say that the more rapidly the useful consumption of zinc takes place, the greater will be the electrical effects produced. But here it must be borne in mind
that if the zinc is being consumed when we are not using the cell or battery, that consumption is sheer waste, quite as much as if we were compelled to burn fuel in an engine whether the latter were doing work or not. For this reason the use of commercial zinc, in its ordinary condition, is not advisable in batteries in which acids are employed, since the zinc is consumed in such, whether the battery is called upon to do electrical work (by placing its plates in connection through some conducting circuit) or not. This serious objection to the employment of commercial zinc could be overcome by the employment of chemically purified zinc, were it not that the price of this latter is so elevated as practically to preclude its use for this purpose. Fortunately, it is possible to confer, on the ordinary crude zinc of commerce, the power of resisting the attacks of the acid (so long as the plates are not metallically connected; or, in other words, so long as the "circuit is broken"), by causing it to absorb superficially a certain amount of mercury (quicksilver). The modes of doing this, which is technically known as amalgamating the zinc, are various, and, as it is an operation which every one who has the care of batteries is frequently called upon to perform, the following working details will be found useful:—
§ 20. To amalgamate zinc, it should first be washed with a strong solution of common washing soda, to remove grease, then rinsed in running water; the zinc plates, or rods, should then be dipped into a vessel containing acidulated water ([§ 17]), and as soon as
bubbles of hydrogen gas begin to be evolved, transferred to a large flat dish containing water. While here, a few drops of mercury are poured on each plate, and caused to spread quickly over the surface of the zinc by rubbing briskly with an old nail-brush or tooth-brush. Some operators use a kind of mop, made of pieces of rag tied on the end of a stick, and there is no objection to this; others recommend the use of the fingers for rubbing in the mercury. This latter plan, especially if many plates have to be done, is very objectionable: firstly, on the ground of health, since the mercury is slowly but surely absorbed by the system, giving rise to salivation, etc.; and, secondly, because any jewellery, etc., worn by the wearer will be whitened and rendered brittle. When the entire surface of the zinc becomes resplendent like a looking-glass, the rubbing may cease, and the zinc plate be reared up on edge, to allow the superfluous mercury to drain off. This should be collected for future operations. It is important that the mercury used for this purpose should be pure. Much commercial mercury contains lead and tin. These metals can be removed by allowing the mercury to stand for some time in a vessel containing dilute nitric acid, occasional agitation being resorted to, in order to bring the acid into general contact with the mercury. All waste mercury, drainings, brushings from old plates, etc., should be thus treated with nitric acid, and finally kept covered with water. Sprague, in his admirable work on electricity, says:—"Whenever the zinc shows a grey granular surface (or rather before this), brush it
well and re-amalgamate, remembering that a saving of mercury is no economy, and a free use of it no waste; for it may all be recovered with a little care. Keep a convenient sized jar, or vessel, solely for washing zinc in, and brush into this the dirty grey powder which forms, and is an amalgam of mercury with zinc, lead, tin, etc., and forms roughnesses which reduce the protection of the amalgamation. Rolled sheet zinc should always be used in preference to cast. This latter is very hard to amalgamate, and has less electro-motive power[8]; but for rods for use in porous jars, and particularly with saline solutions, cast-zinc is very commonly used. In this case great care should be taken to use good zinc cuttings, removing any parts with solder on them, and using a little nitre as a flux, which will remove a portion of the foreign metals."