Sometimes it is desired to coat the body (as a baser metal) with a permanent adherent layer (as of gold or silver); sometimes with a non-adherent layer that, when detached, gives us a 'reverse' impression of the surface on which it was deposited. The processes of electro gilding and electro-silvering come under the first head; those of reproduction of engravings, casts of coins, &c., come under the second head. Sometimes also we employ the process to etch designs on the anode.

In § 11 we shall see that under the above conditions, i.e. when the solution remains unaltered in strength while there is a transference of metal from the anode to the kathode, any single cell however weak can effect the electrolysis (see also Chapter XV.).

As a general rule a single battery-cell with large electrodes gives a compact film of metallic appearance, while a powerful battery, with electrodes too small for the current, gives a spongy non-metallic-looking coating. In a word, for good results the current must not be too dense.

I. Copper-plating.-In this case we wish to coat metals with copper. In more technical books (such as Hospitalier's Formulaire Pratique de l'Electricien' and Sprague's Electricity') the student will find recipes for copper solutions suitable for different purposes, and directions as to current-density, &c., such as will insure the best results. In the present Course we shall give little but the principles of the different methods, and shall therefore give but one recipe for a copper solution. 'A quantity of water is mixed slowly with from one-tenth to one-twelfth its volume of strong H2SO4, and the mixture when cold is saturated with CuSO4.'

As a battery we may use one Daniell's cell of sufficiently large size. This is connected up with two metal rods B and D as

shown, so that conducting bodies hung from B form the kathode, while a copper plate hung from D forms the anode. All wires, &c., which it is desired to protect from action must be coated with wax, or with caoutchouc, or other varnish. If the body is a nonconductor it should be covered with some conducting substance, and care must be taken, by encircling it with a copper wire or by some other means, to make the action uniform over the surface. The bodies to be coated may, more simply, be made to form the negative plate in a Daniell's cell. In this case the substitution of salt and water for battery acid, as the solution acting on the zinc, will make the action slower and the copper-film more compact. If the action is sufficiently slow, the deposit of a film of mm. should take from twelve to twenty-four hours (Hospitalier), though a quicker rate of deposit may give good results.

II. Reproduction of engravings, &c.-If the object to be reproduced be a delicate one, we may proceed as follows. We

will suppose, e.g., that it is required to get a facsimile of one side of a rare coin. First, we take a wax cast (in reverse) of the coin. This wax reverse is then coated with plumbago, surrounded with a thin copper wire so as to distribute the current more evenly, and made to form the kathode in an electrolytic cell. The copper coating will, when removed, reproduce the surface of the coin in question. Or we may make complete casts of bodies and obtain clectro-copper copies or reverses, according as the cast is a reverse or not.

III. Electro-etching.-By covering a copper plate with wax, tracing characters through the wax, and using the plate as an anode, we may etch out designs to any required depth. It is, however, not very easy to obtain any delicate results in this way.

IV. Obtaining designs in relief.-If we use the plate, prepared as in III., as the kathode in the cell, we shall obtain the same designs in relief.

V. Silver-plating. For this purpose we must employ a silver solution, and a silver plate as anode. One recipe for the silver solution is as follows (Hospitalier). Dissolve 150 grammes of

AgNO, in ten litres of distilled water; add 250 grammes of pure KCN; stir until there is complete solution; then filter.'

In silver-plating good results are more difficult to obtain than in copper-plating, and more technical books than the present Course should be consulted if the student intends more than mere illustrative experiment.

§ 11. Polarisation of the Electrodes.-In simple electrolysis, i.e. where no simultaneous decompositions (see § 6) occur, there are two cases to be considered.

I. Where there is no polarisation.-In the cases of electroplating considered above, the condition of the plates and of the solution remains constant. The fact that the kathode increases, and the anode decreases, in thickness during the action obviously does not affect the question of whether there will be a 'back tendency.' Hence, there is no more tendency for the chemical changes to take place in a reverse order after electrolysis has proceeded than there was at first. There is, in fact, no chemical-potential-energy stored up by the electrolytic action; and no tendency, therefore, for the electrolytic cell to drive a reverse current (see Chapter XI. § 10 and Chapter XV. §§ 8 and 9).

Note.-There may be a very slight amount of polarisation owing to the fact that the deposited metal is not of exactly the same quality as the anode plate.

Experiment.-We may pass a current through a Cu | CuSO, | Cu cel) for some time, and then prove the absence of polarisation by connecting the terminals of the cell with a galvanometer, when no current will be observed. Care must be taken that the original current be not so dense as to give simultaneous decomposition of water.

There being no back E.M.F., we can perform such electrolyses with any cell, however weak. (For further on this matter, see

Chapter XV. § 11.)

II. Where there is polarisation.—But in all such cases as that of the decomposition of water, i.e. cases where the condition of the cell changes after electrolysis, we have a decomposition effected by the current against chemical affinities. We have, set free at the electrodes, ions that have a chemical tendency to reunite; and this recomposition can take place most readily by means of a reverse current and a reverse chain of molecular interchanges. We have, in such cases, done chemical work; we have gained chemicalpotential-energy. The cell is now like a battery-cell; and, like a battery-cell, it will tend to drive a current. The cell will have a certain 'electromotive force' depending on the energy of the chemical affinities of the ions; and this E.M.F. must obviously be opposed to that of the original current by which the electrolysis was effected. (For E.M.F. see Chapter XI. § 11, note.)

Such cells are in fact battery-cells; but, as a rule, they will not act as such for a long time, as the supply of ions ready to reunite is limited.

Experiments.—(i.) Grove's gas-battery.-The figure represents several 'cells' for the decomposition of water. It will be noticed that the electrolysed gases are collected separately, and that the platinum electrodes are so long that their upper extremities are always immersed in the gas, however little of this latter there may be.

First, a current is passed through each cell separately until a considerable amount of gas has been set free in each; this amount must be the same in all.

Then these cells are in reality battery-cells. They can be coupled up 'for small internal resistance,' all the kathodes together and all the anodes together; or for E. M. F.,' anode to kathode and so on.

The hydrogen in the one tube and the oxygen in the other tube of each tend to reunite; and it is found that when the external circuit is completed this reunion will take place step by step with the passage of a current, the

action being merely the molecular interchange, explained in § 4, reversed in direction. The current will be in the reverse direction to that by which the electrolysis was in the first instance effected.

Here each cell answers to a Volta's cell in which the zinc plate has been replaced by hydrogen in contact with platinum; the copper plate by oxygen in contact with platinum.

Notes.-(i.) The recombination can occur only when the circuit is closed; and, further, only when there is a platinum plate that is in contact both with the gas and the liquid. This action of the platinum will be familiar to those who have studied elementary chemistry.

(ii.) There will be a current if we replace the platinum-oxygen plate by plain platinum. This is due to the fact that there is always some oxygen dissolved in the dilute acid.

§ 12. Secondary, or 'Storage,' Cells. Any arrangement by means of which chemical-potential-energy may be used up in such a manner as to give us the equivalent energy of an electric current is called a 'battery-cell,' or often more shortly a cell.' Where the cell is put together out of certain materials and then does not require the passage of an electric current before it is fit for use, it is called a primary-cell! All the cells described in Chapter XI. § 11, were of this class.

But where the cell requires the passage of a current before it is ready for use, i.e. where the chemical-potential-energy is the result of electrolysis, in such cases the term 'secondary-cell' or 'storage-cell,' is employed. Such a cell, e.g., is the Grove's gas-cell. The latter term was intended to imply that we 'stored up' elec