surface in consequence of the metallic impurities in it. To do away with this local action, and abolish the wasting of the zinc while the battery is at rest, it is usual to amalgamate the surface of the zinc plates with mercury. The surface to be amalgamated should be cleaned by dipping into acid, and then a few drops of mercury should be poured over the surface and rubbed into it with a bit of linen rag tied to a stick. The mercury unites with the zinc at the surface, forming a pasty amalgam. The iron particles do not dissolve in the mercury, but float up to the surface, whence the hydrogen bubbles which may form speedily carry them off. As the zinc in this pasty amalgam dissolves into the acid the film of mercury unites with fresh portions of zinc, and so presents always a clean bright surface to the liquid.

A newer and better process is to add about 4 per cent of mercury to the molten zinc before casting into plates or rods. If the zinc plates of a battery are well amalgamated there should be no evolution of hydrogen bubbles when the circuit is open. Nevertheless there is still always a little wasteful local action during the action of the battery. Jacobi found that while one part of hydrogen was devolved at the kathode, 33-6 parts of zinc were dissolved at the anode, instead of the 32-5 parts which are the chemical equivalent of the hydrogen.

175. Polarization. The bubbles of hydrogen gas liberated at the surface of the copper plate stick to it in great numbers, and form a film over its surface; hence the effective amount of surface of the copper plate is very seriously reduced in a short time. When a simple cell, or battery of such cells, is set to produce a current, it is found that the current after a few minutes, or even seconds, falls off very greatly, and may even be almost stopped. This immediate falling off in the current, which can be observed with any galvanometer and a pair of zinc and copper plates dipping into acid, is

almost entirely due to the film of hydrogen bubbles sticking to the copper pole. A battery which is in this condition is said to be "polarized."

176. Effects of Polarization. - The film of hydrogen bubbles affects the strength of the current of the cell in two ways.

Firstly, it weakens the current by the increased resistance which it offers to the flow, for bubbles of gas are bad conductors; and, worse than this,

Secondly, it weakens the current by setting up an opposing electromotive-force; for hydrogen is almost as oxidizable a substance as zinc, especially when it is being deposited (or in a "nascent" state), and is electropositive, standing high in the series on p. 85. Hence the hydrogen itself produces a difference of potential, which would tend to start a current in the opposite direction to the true zinc-to-copper current. No cell in which the polarization causes a rapid falling off in power can be used for closed circuit work.

It is therefore a very important matter to abolish this polarization, otherwise the currents furnished by batteries would not be constant.

177. Remedies against Internal Polarization. Various remedies have been practised to reduce or prevent the polarization of cells. These may be classed as mechanical, chemical, and electrochemical.

1. Mechanical Means. If the hydrogen bubbles be simply brushed away from the surface of the kathode, the resistance they caused will be diminished. If air be blown into the acid solution through a tube, or if the liquid be agitated or kept in constant circulation by siphons, the resistance is also diminished. If the surface be rough or covered with points, the bubbles collect more freely at the points and are quickly carried up to the surface, and so got rid of. This remedy was applied in Smee's Cell, which consisted of a zinc and a platinized silver plate dipping into dilute sulphuric acid; the silver

plate, having its surface thus covered with a rough coating of finely divided platinum, gave up the hydrogen bubbles freely; nevertheless, in a battery of Smee Cells the current diminishes greatly after a few minutes.

2. Chemical Means. If a highly-oxidizing substance be added to the acid it will destroy the hydrogen bubbles whilst they are still in the nascent state, and thus will prevent both the increased internal resistance and the opposing electromotive-force. Such substances are bichromate of potash, nitric acid, and chlorine.

3. Electrochemical Means. - It is possible by employing double cells, as explained in the next lesson, to so arrange matters that some solid metal, such as copper, shall be liberated instead of hydrogen bubbles, at the point where the current leaves the liquid. This electrochemical exchange entirely obviates polarization.

178. Simple Laws of Chemical Action in the Cell. We will conclude this section by enumerating the two simple laws of chemical action in the cell.

I. The amount of chemical action in the cell is proportional to the quantity of electricity that passes through itthat is to say, is proportional to the current while it passes.

A current of one ampere flowing through the cell for one second causes 0-00033698 (or) of a gramme of zinc to dissolve in the acid, and liberates 0-000010384 (or) of a gramme of hydrogen.

II. The amount of chemical action is equal in each cell of a battery consisting of cells joined in series.

The first of these laws was thought by Faraday, who discovered it, to disprove Volta's contact theory. He foresaw that the principle of the conservation of energy would preclude a mere contact force from furnishing a continuous supply of current, and hence ascribed the current to the chemical actions which were proportional in quantity to it. How the views of Volta and Faraday are to be harmonized has been indicated in the last

paragraph of Art. 80. These laws only relate to the useful chemical action, and do not include the waste of "local" actions (Art. 166) due to parasitic currents set up by impurities.

LESSON XV. - Voltaic Cells

179. A good Voltaic Cell should fulfil all or most of the following conditions:

1. Its electromotive-force should be high and constant.

2. Its internal resistance should be small.

3. It should give a constant current, and therefore must be free from polarization, and not liable to rapid exhaustion, requiring frequent renewal of the acid.

4. It should be perfectly quiescent when the circuit

is open.

5. It should be cheap and of durable materials.

6. It should be manageable, and if possible, should

not emit corrosive fumes.

No single cell fulfils all these conditions, however, and some cells are better for one purpose and some for another. Thus, for telegraphing through a long line of wire a considerable internal resistance in the battery is no great disadvantage; while, for producing an electric light, much internal resistance is absolutely fatal. The electromotive-force of a battery depends on the materials of the cell, and on the number of cells linked together, and a high E.M.F. can therefore be gained by choosing the right substances and by taking a large number of cells. The resistance within the cell can be diminished by increasing the size of the plates, by bringing them near together, so that the thickness of the liquid between them may be as small as possible, and by choosing liquids that are good conductors.

180. Classification of Cells. Of the innumerable forms of cells that have been invented, only those of first importance can be described. Cells are sometimes classified into two groups, according as they contain one or two fluids, or electrolytes, but a better classification is that adopted in Art. 177, depending on the means of preventing polarization.

CLASS I. WITH MECHANICAL DEPOLARIZATION.

(Single Fluid.)

The simple cell of Volta, with its zinc and copper plates, has been already described. The larger the copper plate, the longer time does it take to polarize. Cruickshank suggested to place the plates vertically in a trough, producing a more powerful combination. Dr. Wollaston proposed to use a plate of copper of double size, bent round so as to approach the zinc on both sides, thus diminishing the resistance, and allowing the hydrogen more surface to deposit upon. Smee, as we have seen, replaced the copper plate by platinized silver, and Walker suggested the use of plates of hard carbon instead of copper or silver, thereby saving cost, and at the same time increasing the electromotive-force. The roughness of the surface facilitates the escape of hydrogen bubbles. By agitating such cells, or raising their kathode plates for a few moments into the air, their power is partially restored. The Law cell, used in the United States for open-circuit work, is of this class: it has a small rod of zinc and a cleft cylinder of carbon of large surface immersed in solution of salammoniac.

CLASS II. WITH CHEMICAL DEPOLARIZATION.

In these cells, in addition to the dilute acid or other excitant to dissolve the zinc, there is added some more