570

DISTRIBUTION OF HEAT IN THE VOLTAIC CIRCUIT. [280.

wire which is in connexion with the last platinum plate of the battery; a piece of boxwood charcoal or of gas coke about the thickness of a cedar pencil is attached to the wire connected with the zinc plate of the battery; the metal for trial is then placed in the little coke crucible, and the current from 20 or 30 pairs of Grove's battery is transmitted through it by means of the charcoal point with which the negative wire of the battery is armed. Alumina, rutile, oxide of iron, and other refractory bodies, may be fused in the voltaic arc, and subsequently volatilized. Despretz, with his powerful battery of 600 cells, in six parallel series of 100 each, saw charcoal obtained from anthracite, from graphite, from sugar, and from oil of turpentin, when moulded into the form of rods, and heated in an atmosphere of nitrogen under a pressure of between 2 and 3 atmospheres, soften, bend, and swell up: in some cases he cemented together the softened fragments into one mass, and the interior of the vessel was lined with a black deposit of what he supposed to have been volatilized carbon. Gassiot has pointed out the remarkable fact, of which no explanation has as yet been given, that the positive wire, or the wire connected with the terminal platinum plate of the battery, becomes much the hotter of the two in this action. This effect is reversed in the case of the secondary current obtained from the Ruhmkorff coil (312), in which the negative terminal becomes the hottest, and from which the dispersion of solid particles almost exclusively occurs.

Favre (Ann. de Chimie, 1854 [3], xl. 293) has arrived at the interesting conclusion that the quantity of heat evolved by the solution of a definite quantity of zinc in any given circuit is lessened in the battery itself, in proportion as heat is evolved at any given point of the circuit, and that heat is lost when motion is produced by the development and exertion of electro-magnetic power. The quantity of heat thus lost agrees very closely with the quantity required by theory if Joule's mechanical equivalent of heat (129) be adopted. A part of the heat is thus converted into mechanical work, as must be the case if the mechanical theory of heat (130) be true. The simple solution of a given quantity of zinc in sulphuric acid, equal in amount to that dissolved in the battery during each experiment, has been found by previous researches to be represented by the number 18682. In these experiments Favre arranged the battery itself in a calorimeter; and in a second calorimeter he placed the conducting wire, which was coiled in such a manner as to be applicable to the production of electro-magnetic action, the amount of which could be measured by its power of raising a weight. He then

281.]

CHEMICAL ACTIONS OF THE VOLTAIC BATTERY.

571

made five series of experiments. In the first of these the current traversed the battery only and a short copper wire; in the second series, it traversed the battery and the conducting wire of the coil, the iron not being included in the coil; in the third series, the metallic core was previously placed in the axis of the coil; in the fourth series, the apparatus for rotation was set in motion by the electro-magnetism developed, but no weight was raised; and in the fifth series, a known weight was lifted to a definite height by the action of the electro-magnet. The results were follows:

No. of
Experiments.

(Conducting coil.)

Heat lost for
Weight raised.

1st. Calorimeter. 2nd Calorimeter.
(Battery.)

Heat Units.
Total.

as

The fifth column gives the total amount of heat measured in 'units of heat' (see p. 284), from which it will be seen to be sensibly equal in each case.

The colour of the light emitted by the different metals when deflagrated between the wires of the battery, is peculiar for each: gold produces a bluish-white light, silver a beautiful green light, copper a reddish white, mercury a brilliant white light tinged with blue; steel burns with brilliant yellow scintillations, zinc with a powerful white light tinged with blue, and lead with a purple light. If these lights be viewed separately through a glass prism, large dark intervals will be seen between a few brilliant streaks of light of different colours and of definite degrees of refrangibility, so that each metal may be recognised by the spectrum that it gives in the voltaic arc (107, 108).

Chemical Actions of the Voltaic Buttery.

(281) Discharge by Convection.-To the chemist, however, the discharge of the voltaic current by the process of convection, is even more interesting than the brilliant phenomena exhibited by the disruptive discharge, since it is in the discharge by convection that the important chemical actions of electricity are displayed.

It has already been explained when describing the voltameter (272), that if the connecting wires of a voltaic battery terminate in platinum plates or wires which are made to dip into acidulated water, decomposition of the liquid takes place, oxygen being

572

LAWS OF ELECTROLYSIS.

[281.

evolved at the surface of one of the platinum plates, while hydrogen escapes at the surface of the other. This important discovery was made in the year 1800, by Nicholson and Carlisle, and the chemical action of the voltaic pile thus revealed, enabled Davy a few years later to decompose the alkalies and earths, which up to that time had been regarded as elements; but by showing their compound nature he at once modified, in an important manner, the views of chemical philosophy which had prevailed up to that period.

In pursuing these experiments on the voltaic decomposition of water, it was soon observed that when copper wires, or the wires of metals which are easily susceptible of oxidation, are employed, gas escapes from one wire only; whilst if platinum or gold wires be used, gas is evolved from both. In the first case, the oxygen combines with the copper or oxidizable metal, and forms an oxide which is dissolved by the acid liquid, and therefore hydrogen alone escapes; in the second case, both gases are evolved, one from either plate; since neither platinum nor gold has sufficient chemical attraction for oxygen to combine with it at the moment of its liberation.

The process of resolving compounds into their constituents by electricity, is termed electrolysis (from 'electricity' and Avots, releasing); a body susceptible of such decomposition is called an electrolyte; and the terminating wires or plates of the battery are called the poles of the battery. The word electrode is also used as synonymous with the pole of the battery, and it implies the door or path (from ödós, a way) to the current by which it enters or leaves the compound through which it is transmitted. The wires connecting the electrodes with the battery were termed by Ampère rheophores (from péos, a stream, and popέw, I carry).

(282) Laws of Electrolysis.-A great variety of bodies admit of being decomposed by electrolysis, but the process is not applicable to all indiscriminately. It occurs under certain definite laws, which may be stated as follows:

i. No elementary substance can be an electrolyte: for from the nature of the operation, compounds alone are susceptible of electrolysis.

ii. Electrolysis occurs only whilst the body is in the liquid state. The free mobility of the particles which form the body undergoing decomposition is a necessary condition of electrolysis, since the operation is always attended by a transfer of the component particles of the electrolyte in opposite directions. Electrolysis is necessarily a process of electrical conduction, but it is conduction

282.]

LAWS OF ELECTROLYSIS.

573

of a peculiar kind; it is totally different from that of ordinary conduction in solids. If an electrolyte be solidified, it instantly arrests the passage of the current; for it cannot transmit the electric current like a wire or a solid conductor: the thinnest film of any solidified electrolyte between the two electrodes suspends all decomposition. Many saline bodies are good conductors when in a fused condition; for example, nitre whilst in a fused state conducts admirably; but if a cold electrode be plunged into the melted salt, it becomes covered with a film of solid nitre, and no current is transmitted until a continuous chain of liquid particles is restored between the plates by the melting of the film; these effects are readily exhibited by including a galvanometer in the circuit. A few partial exceptions to this rule have been observed, and have already been alluded to (278); but in such cases the decomposition is always extremely limited.

:

iii. During electrolysis, the components of the electrolyte are resolved into two groups: one group takes a definite direction towards one of the electrodes; the other group takes a course towards the other electrode. This direction of the ions (as the two groups which compose the electrolyte have been termed, from to going) depends upon the direction in which the chemical actions are going on in the battery itself. The two platinum plates in the decomposing cell may be distinguished from each other in the manner proposed by Daniell. These plates occupy respectively the position of a zinc and of a platinum plate in an ordinary cell of the battery that is to say, if for this decomposing cell an ordinary battery cell were substituted, a rod of zinc would occupy the place of one of the platinum plates, and would be attacked by the oxygen and acid in the exciting liquid of the battery, whilst a plate of platinum or some other conducting metal would occupy the place of the second platinum plate, and would have the hydrogen of the exciting liquid directed towards it. To the plate of the decomposing cell which corresponds to the zinc rod, Daniell gave the name of the zincode, which is synonymous with the anode of Faraday and the positive pole of other writers. To the plate which corresponds to the platinum or conducting metal, Daniell gave the name of the platinode, which is synonymous with Faraday's term of cathode, and with the negative pole of other writers. Oxygen, chlorine, and the acids generally, make their appearance at the zincode in the decomposing cell during electrolysis; whilst hydrogen, alkalies, and the metals are evolved upon the platinode.

This definite direction which the elements assume during

574

DEFINITE DIRECTION OF ELECTROLYSIS.

[282.

electrolysis may be shown by collecting the gas which is evolved over two platinum plates, connected, one with the last platinum, the other with the last zinc plate, of a combination consisting of three or four pairs of Grove's battery. Hydrogen will be collected over the platinode, or the plate in connexion with the zinc end of the arrangement; whilst from the zincode, or plate in connexion with the terminal platinum plate of the battery, oxygen is evolved.

The following experiment further illustrates the definite direction which the components of the electrolyte assume. Let four glasses be placed side by side, as represented in fig. 234, each

divided into two compartments by a partition of card, or three or four folds of blotting-paper, and let the glasses be in electrical communication with each other by means of platinum wires which terminate in strips of platinum foil. Place in the glass No. 1, a solution of potassic iodide mixed with starch; in 2, a strong solution of common salt, coloured blue with sulphate of indigo; in 3, a solution of ammonic sulphate, coloured blue with a neutral infusion of the red cabbage; and in 4, a solution of cupric sulphate. Let the plate, h, be connected with the positive wire, and let a complete the circuit through the negative wire. Under these circumstances iodine will speedily be set free in b, and will form the blue iodide of starch; chlorine will show itself in d, and will bleach the blue liquid; sulphuric acid will be seen in f, and will redden the infusion of cabbage; sulphuric acid will also be liberated in h, as may be seen by introducing a piece of blue litmus paper, which will immediately be reddened; whilst a piece of turmeric paper will be turned brown in a, from liberated potash; in c it will also be turned brown by the soda set free; in e the blue infusion of cabbage will become green from the ammonia which is disengaged; and in g metallic copper will be deposited on the platinum foil.