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ELECTRICAL THEORY OF CHEMICAL ATTRACTION.

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tion of potassic sulphide and one of cupric chloride be mixed, they will decompose each other; the sulphur being negative, will tend to combine with the positive copper, and the positive potassium to unite with the negative chlorine. If the brackets in No. I represent the mode in which the molecules are arranged on the instant of mixture previous to decomposition, those in No. 2 will illustrate the arrangement of the molecules after mutual decomposition has been effected :

(291) Theory of the Electrical Origin of Chemical Attraction. -It has already been remarked (245, 257) that whenever two dissimilar substances, electrically insulated, are first brought into contact, and are then separated from each other, disturbance of their electrical equilibrium is produced; one of the bodies becoming negatively electrified whilst the other acquires a corresponding charge of positive electricity. It is a well ascertained fact, that certain substances, by friction, acquire one kind of electricity more readily than the other; sulphur, for example, when rubbed upon flannel or fur, becomes negatively electric, whilst glass, on the other hand, most readily assumes the positive state. What has been proved to occur when masses of matter are brought into contact was supposed by Davy (Phil. Trans. 1807, 39) to happen also when the molecules of two dissimilar substances are brought within the sphere of mutual activity: he assumed that chemical and electrical attractions depend upon the same cause, acting in one case on particles, in the other on masses of matter' (Phil. Trans. 1826, 389); and all the phenomena of chemical attraction have been referred to the exertion of mutual electrical attraction between the atoms of each substance in the compound. When, for example, chlorine and potassium are united, it is supposed that each atom of chlorine, by contact with an atom of potassium, becomes negatively electrified, whilst the potassium becomes positively excited; a certain portion of the positive electricity from the chlorine uniting with a corresponding amount of negative electricity which is liberated from the potassium, thus producing the light and heat which attends the combination of these two bodies (Berzelius). Supposing each atom of both kinds of matter to be associated with equal quantities of both electricities, and that the two different electricities be represented by

596

THEORY OF ELECTRICAL ORIGIN OF

[291. the signs and, we may represent the potassium and chlorine by symbols; (- K+) indicating an atom of potassium, and (- Cl+) an atom of chlorine. As soon as the two bodies are brought into contact, it is supposed that the chlorine loses a certain amount of positive electricity, whilst the potassium loses an equal quantity of negative electricity, the change being symbolized thus (+ KCl −) and (+). When the potassic chloride is decomposed electrolytically, a quantity of positive electricity is transferred from the positive wire of the battery to the chlorine, and compensates for that which it lost in the act of combining; and when this amount of electricity has been restored, the chloriue no longer has any tendency to remain in combination with the potassium, and hence it is set free upon the positive plate, whilst a simultaneous transfer of negative electricity to the potassium occurs from the negative plate, and the alkaline metal is therefore liberated upon the negative side of the arrangement. The electricity which is set free by the battery is supplied by the action of the sulphion upon the zinc, in the cells of which the battery consists.

The remarkable law discovered by Faraday, that the same current of electricity when transmitted successively through various electrolytes, decomposes each in the proportion of their respective chemical equivalents (282, iv.) adds greatly to the probability of the supposition that electrical and chemical phenomena are due to different manifestations of the same cause. So strong was Daniell's conviction upon this point, that he applied the term current affinity to the voltaic current; since by means of the proper application of conductors, or channels for the force, the chemical attraction of a portion of zinc and sulphuric acid at one point could be transferred to a distant spot, and could there be made to effect an equivalent amount of chemical decomposition upon a different compound. Upon the electro-chemical theory, the chemical equivalent of any substance is that quantity of each body which is associated with an amount of electricity equal to that associated with a given weight of some substance, such as hydrogen, which is selected as the standard of comparison; the proportion of electricity which is associated with a given weight of any substance being inversely as its chemical equivalent, not its atomic weight. Assuming the specific electricity of hydrogen to be represented by the arbitrary number 1000, the following is given by Daniell (Introd. to Chem. Phil. 1843, 2nd Ed. 687) as an approximate table of the specific electricity (or quantity of elec

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CHEMICAL ATTRACTION-OBJECTIONS.

597

tricity associated with equal weights) of a few of the more important elements and compounds :

Ingenious, however, as is the electrical theory of chemical attraction, it must be admitted that it is far from being free from objection and difficulty when an attempt is made to apply it to all cases of chemical action. It has been already stated that a very large number of bodies exist which are not susceptible of electrolysis. Indeed, the chief classes of electrolytes are: 1, binary, compounds of the non-metallic elements with the metals, such as the oxides, chlorides, iodides, bromides, and fluorides; 2, compounds of the metals with bodies like cyanogen, such as the cyanides and the sulphocyanides; and 3, compounds of the metals with the oxions or radicles of the oxyacids, such as the nitrates, sulphates, borates, carbonates, acetates, tartrates, &c. Now, as long as a compound consists of two elements only, if it be decomposed at all, there is no difficulty in anticipating the result of the voltaic action the electro-negative element will appear at the zincode, and the electro-positive element at the platinode; yet there are compounds consisting of two elements only which are not electrolysable, such, for instance, as pure water, and sulphur chloride (S2Cl2). If their particles be united by electric opposition, why

:

*In the experiments of Lapschin and Tichanowitsch (Phil. Mag. 1861 [4], xxii. 308) a very numerous series of Grove's battery, amounting to 900, and in some instances even to 950 pairs, produced no effect on absolute alcohol, ether, valeric acid, oil of turpentin, carbonic disulphide, or fused boracic anhydride fusel-oil was scarcely acted on. Bleekrode (Proc. Roy. Soc. 1876, xxv. 322) has experimented with Mr. De La Rue's battery of 8040 cells on liquefied ammonia, carbonic disulphide, benzol, stannic chloride, liquefied carbonic anhydride, liquefied hydrochloric acid, cyanogen, and zinc-ethyl. The ammonia was decomposed with the formation of a deep blue colour, which rapidly disappeared when the current was stopped, and zinc-ethyl also showed signs of electrolysis. The other liquids were unaffected.

598

ELECTROTYPE, OR VOLTATYPE.

[291. should they not yield to the current? In the case of more complex bodies, such, for example, as argentic nitrate or plumbic borate, it is not possible, à priori, to say how the compound would yield under the electric influence. It is quite clear in the case of a salt, that the power which holds together the two ions of the salt in the form of two iso-electric groups (or groups of equal electric energy), must be of a different order from that which holds the elements of its component ions in combination. The tie which binds together argentic nitrate as AgNO3, must be of an order different from that which unites the elements of nitrion (NO3) together. Sodic sulphate, again, as an electrolyte is separable into Na, and SO. But neither nitrion nor sulphion can exist in the separate form; how can they become associated under electric influence? Again, sulphuric anhydride (SO) is not an electrolyte when fused: the same thing may be said of fused boracic anhydride; and examples of this kind might be multiplied almost without limit. Why, if chemical attraction be due to the exertion of electric action, should certain bodies be readily decomposable by the voltaic current, and why should others of less complex composition resist it entirely? At present, no hint appears to have been given which offers any clue to the solution of these questions.

Practical Applications of Electrolytic Action.

(292) Electrotype, Voltatype, or Galvano-Plastics.-Shortly after Daniell had invented his constant battery, he observed that when copper was deposited upon a plate of platinum, it furnished a coherent sheet in which the lines and irregularities on the surface of the platinum were faithfully reproduced upon the deposited copper; but he made no practical application of the observation. In the year 1839, Jacobi, of St. Petersburg, announced that he had discovered a method of making exact copies of a metallic surface in copper by means of the voltaic battery, and shortly afterwards Messrs. Spencer and Jordan, who had each independently arrived at a similar result, published the methods which they had employed for the attainment of this object. The processes thus disclosed were so simple and easy of execution that they were immediately repeated with success; and in the following year Mr. Elkington in England, and M. Ruolz in France, began to apply the voltaic battery on an extensive scale to the arts of plating and gilding. Since this period the voltaic battery has been most extensively employed as a means of depositing not only copper, gold, and silver from their solutions, but zinc, tin, and lead,

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ELECTROTYPING OF COPPER PLATES.

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and occasionally platinum and nickel: many other metals have also, for particular purposes, been reduced from their salts by its means.

For the deposition of metallic copper, a solution of the sulphate of this metal is employed, but the mode of using it varies with the object in view. Suppose that it be desired to obtain a copy of an engraved copper plate; a wire or slip of copper having been soldered to the plate for the purpose of facilitating its connexion with the battery, the back of the plate is covered with a resinous varnish, by which means this surface is electrically insulated from the solution, and it is thus protected from any deposit of reduced metal. The plate thus prepared is connected with the negative electrode of a voltaic battery, consisting of three or four of Smee's or Daniell's cells, and immersed vertically in a bath consisting of a solution of cupric sulphate (sulphate of copper) slightly acidulated and not quite saturated. A sheet of copper, equal in size to the one to be copied, is suspended parallel to the latter in the liquid, and connected with the positive electrode of the battery; an immediate decomposition of the solution ensues; metallic copper is deposited upon the entire surface of the negative plate, in the form of a coherent, continuous sheet, and an amount of copper corresponding closely to this is dissolved from the positive plate, so that the liquid remains coustantly charged with a quantity of cupric sulphate approximately equal to that originally employed. At the commencement of the operation, care must be taken to ascertain that the deposit occurs uniformly over the whole surface of the negative plate, for if any portion of it be soiled by grease or resinous matter, the copper will not be thrown down upon those parts; when once the deposition has commenced uniformly, it goes on without difficulty. If the plates be suspended vertically, the solution should be frequently agitated, for unless this precaution be taken, the liquid around the negative plate becomes impoverished, whilst that around the positive plate becomes unduly saturated with the copper salt (287); currents are then produced in the liquid, owing to its unequal density, and they occasion the formation of vertical grooves and striæ upon the back of the sheet of deposited metal. This inconvenience is sometimes obviated by supporting the two plates in the bath in a horizontal position, the negative plate being undermost; the positive plate must in this case be enveloped in flannel, in order to prevent the small particles of metal, which are constantly being detached from it, from falling upon the lower plate, and interfering with the regularity of the deposition.