§ 21. Another and very convenient mode of amalgamating zinc, specially useful where solid rods or masses of zinc are to be used, consists in weighing up the zinc and setting aside four parts of mercury (by weight) for every hundred of the zinc thus weighed up. The zinc should then be melted in a ladle, with a little tallow or resin over the top as a flux. As soon as melted, the mercury should be added in and the mixture stirred with a stick. It should then be poured into moulds of the desired shape. This is, perhaps, the best mode of amalgamating cast zincs.
§ 22. Some operators recommend the use of mercurial salts (such as mercury nitrate, etc.) as advan
tageous for amalgamating; but, apart from the fact that these salts are generally sold at a higher rate than the mercury itself, the amalgamation resulting, unless a very considerable time be allowed for the mercuric salts to act, is neither so deep nor so satisfactory as in the case of mercury alone. It may here be noted, that although the effect of mercury in protecting the zinc is very marked in those batteries in which acids are used as the exciting fluids, yet this action is not so observable in the cases in which solutions of salts are used as exciters; and in a few, such as the Daniell cell and its congeners, the use of amalgamated zinc is positively a disadvantage.
§ 23. If, having thus amalgamated the zinc plate of the little battery described and figured at [§ 9], we repeat the experiment therein illustrated, namely, of joining the wires proceeding from the two plates over a suspended magnetic needle, and leave them so united, we shall find that the magnetic needle, which was originally very much deflected out of the line of the magnetic meridian (north and south), will very quickly return near to its old and normal position; and this will be found to take place long before the zinc has been all consumed, or the acid all neutralised. Of course, this points to a rapid falling off in the transmission of the electric disturbance along the united wires; for had that continued of the same intensity, the deflection of the needle would evidently have remained the same likewise. What, then, can have caused this rapid loss of power? On examining (without removing from the
fluid) the surface of the copper plate, we shall find that it is literally covered with a coating of small bubbles of hydrogen gas, and, if we agitate the liquid or the plates, many of them will rise to the surface, while the magnetic needle will at the same time give a larger deflection. If we entirely remove the plates from the acid fluid, and brush over the surface of the copper plate with a feather or small pledget of cotton wool fastened to a stick, we shall find, on again immersing the plates in the acid, that the effect on the needle is almost, if not quite, as great as at first; thus proving that the sudden loss of electrical energy was greatly due to the adhesion of the free hydrogen gas to the copper plate. This peculiar phenomenon, which is generally spoken of as the polarisation of the negative plate, acts in a twofold manner towards checking the electrical energy of the battery. In the first place, the layer of hydrogen (being a bad conductor of electricity) presents a great resistance to the transmission of electrical energy from the zinc plate where it is set up to the copper (or other) plate whence it is transmitted to the wires, or electrodes. Again, the copper or other receiving plate, in order that the electric energy should be duly received and transmitted, should be more electro-negative than the zinc plate; but the hydrogen gas which is evolved, and which thus adheres to the negative plate, is actually very highly electro-positive, and thus renders the copper plate incapable of receiving or transmitting the electric disturbance. This state of things may be roughly likened to that of two exactly equal and level tanks, Z
and C, connected by a straight piece of tubing. If Z be full and C have an outlet, it is very evident that Z can and will discharge itself into C until exhausted; but if C be allowed to fill up to the same level as Z, then no farther flow can take place between the two.
It is, therefore, very evident that to ensure anything like constancy in the working of a battery, at least until all the zinc be consumed or all the acid exhausted, some device for removing the liberated hydrogen must be put into practice. The following are some of the means that have been adopted by practical men:—
§ 24. Roughening the surface of the negative plate, which renders the escape of the hydrogen gas easier. This mode was adopted by Smee in the battery which bears his name. It consists of a sheet of silver, placed between two plates of zinc, standing in a cell containing dilute sulphuric acid, as shown at [Fig. 5].
