If the union of the electricities collected in the poles has been effected by connecting the poles by a perfect conductor, and the connection is afterwards broken, the two electricities again accumulate in the poles with their former tension. In piles with moist semi-conductors, this effect takes place almost instantaneously; in those with dry semi-conductors, often very slowly. So long as the poles remain connected by a good conductor, the positive and negative electricities rapidly generated in the battery continually recombine within the conductor. The quantity of electricity thus neutralized in a given time depends on the circumstances above noticed.

The difference between the effects of the voltaic pile and those of the electrical machine consists in the two following points: 1. By means of the latter, a large quantity of electricity may easily be accumulated in a body of small dimensions; in consequence of which, the electricity acquires a high tension or a strong tendency to combine with electricity of the opposite kind, and makes its way through non-conductors, such as the air, in the form of a spark. On the other hand, the tension of the electricity developed by the contact of two metals is so extremely small, that, according to Children, it requires no fewer than 1250 zinc and copper plates to give such a tension to the electricity accumulated at the poles, as will enable it to pass in the form of a spark between two platinum wires terminating in points, placed at an interval not exceeding inch, and connected with the poles of the battery.-(2.) But a small voltaic battery generates in a given time a much greater quantity of electricity than a large electrical machine,-provided the electricities are conducted away as fast as they are generated. The electrical machine, therefore, is best adapted to the production of those effects which depend on great electrical tension, such as the penetration of air, glass, and other insulators; the voltaic battery, especially when large plates are used, is most efficacious in producing those effects which depend on the combination of large quantities of electricity in a given time,-e. g., an elevated temperature in the conductors in which the combination takes place, and the decomposition of chemical compounds.

Since an increased number of plates augments, not the quantity, but only the intensity of the current, it follows that the number of pairs united in the battery ought to be different, according to the conducting power of the body through which the current has to pass. A greater number of pairs than that required for completely overcoming the resistance, is of no avail-and may even diminish the quantity of the current, any among them act less powerfully than the rest. Hence a single pair is proper when the current has to pass through a thick wire only (as for magnetic action or for fusing), a small number of pairs when it has to traverse a long thin wire, a larger number when watery liquids are placed in the circuit, and a still larger number when the liquids are divided into separate portions by interposed plates. (De la Rive.)

if

The greater the number of pairs, the sooner does the action diminish; so that a battery of a small number of pairs is stronger after a time than one containing a large number. (De la Rive.)

Strongly acting pairs must not be united with such as act weakly, e. g., zinc and copper with copper and platinum; or pairs acted upon by fresh acid with pairs subjected to the action of acid which has been used and is therefore saturated; neither should large pairs be united with small The addition of pairs which act less powerfully [though it increases the tension] diminishes the quantity of the current considerably. If 40

ones.

pairs with fresh acid produce a current capable of liberating 8.4 cubic inches of detonating gas in a minute by the decomposition of water, the quantity obtained in the same time is reduced to 11 cubic inches when these 40 pairs with fresh acid are connected with 10 pairs acted upon by acid previously used. (Faraday.)

Materials of the Battery.

Zinc, either common or amalgamated, is almost always employed as the positive metal of the battery. But since common zinc is subject to ordinary chemical action, and likewise-on account of alloys mixed with it-to local galvanic action; and since these actions continue even while the circuit is open-a great deal of zinc and acid are uselessly consumed. By the use of amalgamated zinc, first introduced by Sturgeon, this loss is avoided. With this kind of zinc, however, the current is soon reduced to or of its original strength, because no gas is evolved on the zinc, and consequently the acid does not get well mixed; but on breaking contact for a while, the current regains its former power.Rolled zinc plates are preferable to those of cast zinc, which are less pure. New plates act better than such as have been used two or three times, perhaps because the alloys come out more prominently as the surface dissolves, and thus give rise to local galvanic action. This deterioration of zinc plates by use soon reaches its limit in the case of rolled zinc; but with cast plates it goes on continually. The more smoothly the plates are rubbed, the better do they work. (Faraday.)

For the negative metal-copper, iron, lead, silver, platinized silver, platinum, graphite, charcoal, or peroxide of lead may be used.

In a battery of iron, platinum, and dilute sulphuric acid, only a small quantity of hydrogen gas is evolved upon the platinum; but if the iron plates used in this battery are united with zinc plates, nearly twice as much hydrogen is evolved upon the iron, as would be given off, if the same zinc plates were connected with platinum. Fresh iron plates, on the contrary, do not act more strongly than platinum. (Daniell.)

When the surface of the negative metal is twice as great as that of the positive, and a plate of the latter is interposed between two plates of the former, the quantity of the current is greatly increased (p. 380). (Hare, Faraday.) The quantity is also greater, the thinner the stratum of liquid between the zinc and the negative metal (p. 377). (Faraday.)

A peculiarly strong action is produced by silver or plated copper, on which platinum is precipitated. A rough surface is first imparted to the silver by treating it for a short time with strong nitric acid; it is then immersed in dilute sulphuric acid, to which a little chloride of platinum has been added, and connected with zinc immersed in dilute sulphuric acid contained in a porous tube, this tube dipping into the first-mentioned liquid. The platinum is precipitated on the silver in the form of a black powder. (Smee.)

Of carbonaceous substances, the most serviceable are plumbago and hard gas-coke; then porous coke and box-wood charcoal. These, however, soon lose their power, in consequence of becoming filled with hydrogen: but if the hydrogen be removed by immersing them in a solution of sulphate of copper, which likewise produces a precipitation of copper upon and within them, a powerful battery may be made with them. (Smee.)

A mixture of coal and coke, strongly ignited, forms a carbonaceous mass, which acts nearly as well as platinum, and is well adapted for use

on account of its tenacity. (Bunsen, Ann. Pharm. 38, 211.) (Comp. p. 392.)

For liquids, the following may be used: spring-water, aqueous solutions of common salt, sal-ammoniac, nitre, alum, sulphate of copper, bisulphate of potash, and other salts, aqueous ammonia or potash, and dilute sulphuric, hydrochloric, or nitric acid, or a mixture of dilute sulphuric and nitric acids.

Water gives a continuous current of small quantity and attacks the zinc but very little. With saline solutions, the current is at first abundant, but soon diminishes (p. 377). Sulphate of copper is the only salt that gives a more permanent current; it also acts without evolution of gas.

A mixture of water, sulphuric acid, and nitric acid (e. g., 100 measures of water to 4.5 oil of vitriol and 4 nitric acid) possesses this advantage, that it not only excites an intense and abundant current, but that neither hydrogen nor nitric oxide gas is evolved from it, because the hydrogen evolved from the water combines with the nitrogen set free from the nitric acid, and forms ammonia. (For Fyfe's experiments on the power of different circuits, vid. Pogg. 43, 228.)

When dilute sulphuric and nitric acid are used together, instead of dilute sulphuric acid alone, in a battery of ten pairs of amalgamated zinc and platinum, the quantity of the current (measured by the Voltameter) is nearly trebled, and the quantity of hydrogen gas evolved in the different cells is small and unequal; but in five minutes the current ceases almost entirely, and is but imperfectly restored by opening the circuit. When more nitric acid is added, the evolution of hydrogen ceases entirely in some cells, and almost wholly in others: the current is strengthened, but diminishes rapidly. If when the circuit has been closed for thirty hours, and the current has almost wholly ceased, the platinum plates be removed and replaced by new ones, the battery will again act as strongly as at first, but only for a short time. The platinum plates, which have become inactive, are not completely restored by ignition, polishing, or boiling with strong solution of potash; but they recover their activity entirely by boiling in nitric acid. For they are covered with zinc partly crystalline, partly of a warty texture, the coating even extending, though in smaller quantity, to the side which is turned away from the zinc. The rapidity with which this coating is formed, increases with the quantity of zinc dissolved in the liquid; it is therefore greatest when unamalgamated zinc is used.-Every time the circuit is broken, the zinc deposited upon the platinum dissolves again, provided a sufficient quantity of free acid be present. When the acid liquid holds a quantity of copper in solution, that metal is first deposited on the platinum, and afterwards the zinc, but with less facility. But when a larger quantity of copper is contained in the liquid, the copper is precipitated on the amalgamated zinc, and causes an evolution of hydrogen upon it. The addition of nitric acid to the dilute sulphuric acid assists the action by diminishing the evolution of hydrogen gas on the platinum, inasmuch as it gives rise to the production of ammonia [by which however the acid is so much the more quickly saturated]. (Daniell.)

Remarkably powerful currents are produced by the proper combination of two metals with two liquids (p. 389).

With plates of given surface, the quantity of the current varies according to their nature and that of the liquid, as follows: Amalgamated zinc, dilute sulphuric acid, copper: 0·19; amalgamated zinc, dilute sulphuric acid, platinized silver (according to Smee) : 0.29;-amalgamated zinc,

dilute sulphuric acid, iron (according to Sturgeon): 0.30 or less;-zinc dilute sulphuric acid, sulphate of copper, copper (according to Daniell) : 0.65;-amalgamated zinc, dilute sulphuric acid, concentrated nitric acid, platinum (according to Grove): 109. (Joule.)

1. Batteries with two Metals and two Liquids.

Daniell's Constant Battery. (App. 25.) An ox-gullet closed at the bottom (or connected with a bent tube for carrying off the spent acid) contains dilute sulphuric acid and a rod of amalgamated zinc, and is fixed upright in a cylindrical copper vessel filled with a solution of sulphate of copper. The acid is either constantly renewed by causing it to flow into and through the bent tube, or occasionally by pouring it in when wanted. The solution of sulphate of copper is maintained at a constant strength by means of portions of the pulverized salt placed in sieves at the upper part of the liquid. From 10 to 20 simple circuits of this kind are united (the copper vessel of the first with the zinc cylinder of the second, the copper vessel of the second with the zinc cylinder of the third, and so on, the connection being made by metallic conductors).-This battery evolves no hydrogen gas, even when the ends are unconnected: the copper of the solution is gradually deposited, when the circuit is closed, on the inner surfaces of the copper cylinders. When the current is made to pass through acidulated water, the quantity of detonating gas evolved is greater in the first quarter of an hour than in any subsequent equal interval; but from that time the current remains perfectly uniform,-so that when the circuit has been closed for four hours, the quantity of detonating gas evolved in a given time on passing the current through acidulated water, is the same as before. The amount of pure chemical action in this battery is very small. For 188 48 cub. in. of detonating gas evolved in the decomposition of water by a ten-pair battery, 93.3 grains of zinc are dissolved in the same time from each zinc plate (therefore 933 gr. for the whole ten plates); that is to say, only 7.2 gr. more than the quantity required by the stoichiometrical proportion of water decomposed to zinc dissolved (9: 32.2).—Whilst the ten-pair battery, when the zinc is immersed in a mixture of 100 measures of water and 4 measures of oil of vitriol, liberates 38 cub. in. of detonating gas in 5 minutes, it gives 2.1 cub. in. when nitric acid is added to the sulphate of copper solution, and 4.2 cub. in. on the addition of nitric acid to the sulphuric: the latter quantity however soon sinks again to 38 cub. in. Hence the addition of nitric acid is useless. With a mixture of 100 measures of water and 9 measures of oil of vitriol, the quantity of obtained in 5 minutes is 5.5 cub. in.; and with 100 water and 12.5 oil of vitriol, it amounts to 11 cub. in.-When the circuit is kept closed for five hours without renewal of the acid, the quantity of detonating gas evolved in 5 minutes sinks from 2·7 to 2·4 cub. in., and after 24 hours to 0-3 cub. in.; the acid is then found to be completely saturated with oxide of zinc. If half the liquid be now replaced by fresh dilute sulphuric acid, the quantity of detonating gas evolved in 5 minutes rises, not merely to the original quantity of 2.7, but to 4.2 cub. in.; and the battery retains this strength for 4 hours, because the solution of sulphate of zinc mixed with the liquid increases its conducting power. The action remains the same when the zinc cylinders are reduced to one-fourth of their former length. The current is not increased when the copper surface is extended by the introduction of a number of slips of copper.-A twenty-pair battery

gas

makes a platinum wire 8 inches long and of an inch thick, incandescent in the air.—If a zinc cylinder in one of the cells be replaced by a thick platinum wire, pure oxygen gas is evolved upon it, 84 cubic inches in an hour. (Daniell.)

Jacobi (Electrotype, Petersburgh, 1840) places a plate of lead or of thick copper rolled up into a cylinder (not soldered, in order that the precipitated copper may be removed by unbending it) in an oblong rectangular vessel of copper or lead. This cylinder touches three sides of the vessel. The space between it and the fourth side serves to receive a sieve-bottomed trough placed in its upper part and filled with pounded sulphate of copper, in order that the solution which drains through the bottom of the trough may supply the loss of that salt resulting from the galvanic action. Within the lead or copper cylinder is placed another cylinder made of the thinnest possible earthenware, filled with dilute sulphuric acid, and containing a zinc cylinder either solid or hollow. The liquid is withdrawn by means of siphons of peculiar construction; at all events, the acid liquid must be removed from time to time as it becomes saturated with zinc. The earthenware cylinders are soaked in water every six or fourteen days, in order to re-open their pores.

Instead of animal membranes, on which copper becomes precipitated, Mullins uses a hollow cylinder of sycamore or other white wood closed at the bottom; he also uses a solution of equal parts of water and saturated solution of sal-ammoniac in plate of dilute sulphuric acid. (For the other alterations, vid. Phil. Mag. J. 15, 37.)

Spencer (Pogg. 51, 372) substitutes for the ox-gullet in Daniell's battery a cylinder of thick brown packing paper, modelled upon a wooden cylinder, fastened with sealing wax throughout its whole length by means of a hot iron, and closed at bottom with a thin round plate of wood which has a circular groove cut in it, so that it may be tied with a string to the paper cylinder. Solution of Glauber's salt or sulphate of zinc is used instead of dilute sulphuric acid.

Or:-a glass cylinder outside; within this, a cylinder of thin sheet lead or the lead of which the Chinese tea-canisters are made, with numerous vertical folds, so that it has a starry appearance and presents a large surface. Within this again is placed a cylindrical vessel of unglazed earthenware; and this contains a cylinder of zinc surrounded with sulphate of soda or sulphate of zinc. The lead immersed in the solution of sulphate of copper soon becomes covered with copper, and acts like that metal.

Grove's Battery. (App. 26.) A porcelain trough is divided by partitions s, also of porcelain, into four cells, a, b, c, d. Each cell contains a box of porous earthenware. Between the surfaces of the cell and the box is placed a platinum plate pt, passing downwards on one side of the cell and upwards on the other, and in the earthenware box, an amalgamated zinc plate z, connected with the platinum plate of the second cell by thick wires screwed on to the zinc and the platinum, &c. The box i is filled with water containing one-fifth of its bulk of oil of vitriol, or one-half its bulk of hydrochloric acid; and the space containing the platinum is filled with a mixture of equal parts of oil of vitriol and strong nitric acid. The apparatus is fitted with a cover containing lime, in order to absorb the nitrous vapours which are evolved.-The apparatus contains eight ounces of liquid. It must not be left in action more than half an hour, because it becomes too hot. (Grove. Vid. p. 391.)

An instrument of this kind furnished with only single platinum plates -each of the four zinc and platinum plates having a surface of 14 square