of battery is altogether unsuitable for manufacturing purposes, even when amalgamated scrap zincs are used; and, as regards the calculation, it is not easy to see that, while a pound of copper, dissolved from the positive electrode, originally costs 25 cents, it could, notwithstanding, be deposited by the destruction of 1 lb. of zinc, not including acid, etc., at the expense of only 25 cents. It ought always to be remembered, that, for manufacturing purposes, the surface upon which the metal is to be deposited in general amounts to several square feet. The article may be, for example, a large ornamental vase, having four square feet of surface. An odds-and-ends battery, or an iron single pair battery would be too weak. To deposit, with a separate battery, upon a surface such as that of the vase, it requires two or three pairs of plates to give what we may call economical power.

RECOVERY OF MERCURY FROM WASTE ZINC.-The general practice of manufacturers, when the scraps of zinc become small, is either to treat them as referred to at page 512, to distil the mercury from the zinc, or to sell the scraps to parties who do distil them. This is done by putting the scraps into an iron retort, subjecting it to a red heat, and allowing the beak of the retort to pass into a condenser, which has a tube dipping into water. The mercury distils over, and condenses in the water. The zinc left in the retort is found to be so impure as not to be fit to melt and roll again, but it may be used in the composition of common brass. Mr. De la Rue, in a communication to the Chemical Society,* gives the results of several analytical experiments upon scrap zinc. Before distillation the scraps usually give the following results in 100 parts:

The composition of the zinc left after distillation is given as

COMPOUND CELL PROCESS.-Another method of economizing power was proposed in what is termed the compound cell system, by which it was said that the electricity passing through a series of cells would be able to produce the same quantity of work in every celi with no more cost. This plan may be stated thus:

A is a Smee's battery; the wire z is conducting the electricity to

* Memoirs and Proceedings of the Chemical Society, vol. ii. page 293.

the compound trough which is composed of a series of water-tight cells, as a a, and is connected with a piece of copper c, forming a positive electrode; in the same cell, and facing this electrode, is a medal, connected by a copper wire to a piece of copper placed in the second cell, opposite which is another medal connected in the Fig. 575.

X X X X

same manner with another piece of copper, and so on through the series, which terminates with a medal attached to the wire of the battery. The electricity from the battery passes through all these cells, and reduces its equivalent in each cell. Thus the reduction of 32 grains of zinc in the battery would deposit 32 grains of copper multiplied by 6 times, or as many times as there are cells.

This is correct in principle, and at first sight seems to be exceedingly economical; but it is not so, for every cell adds so much to the resistance of the current, that intensity batteries must be used; so that, supposing we have a compound cell of six divisions, in which are placed six separate medals, it would require a battery of six pairs of plates to give intensity sufficient to overcome the resistance, and the same number of medals could be made of the same weight by six separate zincs, and in less than half the time they could be made by this arrangement, and with a less destruction of zinc. For large operations, where the articles receiving the deposits and the electrode are necessarily a good way apart, the process is altogether impracticable in a commercial point of view. This is one of the remarkable instances where theoretical possibility and commercial economy are at variance.

EFFECTS OF RESISTANCE.-At page 551 we mentioned, that if a single cell deposits 100 grains in a given time, and it be converted into a battery having the two electrodes in a solution of sulphate of copper, there will only be deposited in the same time 30 grains. This is caused by the extra resistance which the solution between the two electrodes, in the decomposition cell, offers to the passage of the electricity, the amount of which corresponds to the amount deposited the latter depending upon the former.

If we take two small plates of copper and zinc amalgamated, and place them in dilute sulphuric acid, in contact, but not so close as to prevent the gas evolved from the copper plate to escape, aud allow them to remain until there have been dissolved from the zing

100 grains, and we call this the measure of the maximum amount of electricity which that surface of zinc and copper can give out in the time taken to dissolve the 100 grains: then, if the two metals in the acid be separated one inch, being connected by a wire or slip of copper above the liquid, and kept in action the same length of time as the former, there will be dissolved from the zinc only about 56 grains. If the wire in connection with the zinc and copper be extended and cut in the middle, and have a piece of copper attached to each of the same size as the zinc plate in acid, and these be placed in another vessel containing a solution of sulphate of copper (as Fig. 544), and put an inch apart, and the whole kept in action the same length of time as before, it will be found in this case that only 10 grains of zinc are dissolved. From these experiments we see that the resistance of the one inch of acid between the zinc and copper in the battery, and the one inch of solution of sulphate of copper in the second or decomposition cell, is 90 or nine-tenths-only yielding one-tenth of the electricity which the zinc and copper are capable of giving.

INTENSITY. If we now take another zinc and copper plate of the same size as the former, and arrange them in the acid solution, and connect them with the copper plates in the decomposition cell,

as shown in Fig. 544, and keep them in action the same length of time as in the former experiments, there will be dissolved from the zinc about 19 grains, and deposited upon the copper plate attached to the zinc in the decomposition cell 18 grains of

copper.

If three zincs and coppers be arranged as described and placed in the acid, there will be dissolved from the zinc plate 26 grains, and deposited upon the copper 25 grains. If six pairs zinc and copper be arranged as above and placed in acid, there will be deposited 36 grains of copper, which we will also take as the measure of what is dissolved from the zine; and if nine pairs of zinc and copper be used, there will be deposited 43 grains, and so on until the quantity dissolved from each zinc, or deposited on the copper plate be 100, equal to that obtained by the close contact of the zinc and copper in acid, which will require upwards of 30 pairs of zinc and copper. It must be borne in mind that the same quantity of zinc will be dissolved from every plate in the arrangement. Thus, in nine pairs where 43 grains were deposited, there would be dissolved from every zinc in the battery 43 grains.

It will now be apparent that the use of several pairs in the battery is to overcome resistance, by which quantity is gained at the same time up to a given point; but quantity gained by this means Is expensive. The 10 grains deposited by the single pair of

zinc and copper only required 10 grains of zinc, but the 43 grains by the nine pairs would require 405 grains of zinc to be dissolved.

RELATIVE INTENSITY OF BATTERIES.-Different batteries have different degrees of power to overcome resistance-greater intensity. The following experiments will illustrate this: A single pair of a Wollaston's, Smee's, and Grove's batteries were fitted up as nearly equal in circumstances as the different arrangements would allow each exposing the same surface of zinc, and connected with electrodes placed in a solution of sulphate of copper, first 1 inch, then 2 inches, 3 inches, and 4 inches apart-half an hour in each. They were then reversed, beginning with the electrodes at 4 inches and coming to 1 inch. These experiments were repeated several times, and a mean of the whole taken. The results were:

From this it will be seen that Wollaston's stands lowest in intensity, which is more apparent as the distance of the electrodes is increased. Smee's is one-third more than Wollaston's at 1 inch, and one-half more at 4 inches; while Grove's is three and a-half more than Wollaston's, and two and a-half more than Smee's at 1 inch, but four and a-half more than Wollaston's and three more than Smee's at 4 inches. If we take the mean of these results as a comparison of batteries, their value will stand as under:

One of Grove's equal to three of Smee's,

and to three and three-fourths of Wollaston's.

The following table gives the results of different batteries, arranged in series, kept in action the same length of time, namely, one hour. The battery plates were very small, the electrodes twice the size of the battery plates:

This table gives results approaching to and in principle the same as the others: it will be observed that one pair of Grove's is equal to nine pairs of either Wollaston's or Smee's. It is also worthy of remark, that Grove's increases slowly in quantity above four pairs, the intensity being sufficient at four pairs to overcome the resistance offered to the current of electricity. For ordinary electrotyping, intensity arrangements are unnecessary, except where the article upon which the deposit is being made is of such a character as will not allow the positive electrode to be brought close to it, or when there are deep cut objects, or any circumstance that increases distance and necessitates power to overcome resistance.

MODE OF SUSPENDING OBJECTS FOR COATING.-In beginning to operate in the art of electrotyping, the student often pauses, and asks the question, What is the best position in which a medal should be hung in the solution? Convenience has brought into general practice the suspending of it perpendicularly in the solution, having the

Fig. 577.

positive electrode or pole facing it in a parallel direction; but to this method there are some objections. If, for instance, the porous diaphragm, or single-cell system be used, for obtaining the medals, it is found that upon the lower portion of the medal the deposition is much thicker than upon the upper portion. Indeed, when even ordinary attention is not paid, the lower part becomes not only thicker, but studded over with round nodules of copper, or with lines composed of these nodules, while the upper part remains thin, and is covered over with what is termed the sandy deposit copper, in dark brown grains, capable of being rubbed off with the slightest friction. No doubt this is in a great measure prevented by agitating the solution; but it is inconvenient, and requires constant attention.

If a separate battery is used, and the deposition of the medal is effected in a separate vessel, by having a copper positive electrode, the same inconvenience takes place to a greater or less extent, according to the distance at which the two poles are placed. These inconveniences are known to all electrotypists, and the cause is ascribed to the different densities of the solution. The reason why the solution becomes of different densities is easily understood in the single cell process, there being no copper pole to maintain the strength of the solution; as it becomes exhausted of copper by the deposition, the lighter portion floats on the top, and the heavier portion remains below; and although crystals of sulphate of copper be suspended in the solution, as they dissolve they sink by their gravity, and cause a flow upon the lower portion of the medal, and consequently a much more powerful deposit. But why the same should take place with a separate battery, where there is a positive electrode of copper being dissolved, just in proportion to the copper extracted from the solution by the medals, was for a long time not known.