no matter how small it is, it causes the atoms, when liberated as usual, to tend in one direction, viz., along the lines of force. Hence the collection of the ions at the electrodes, where they wrll separate it the electrornotive torce be sufficient to prevent them reacting and again reconibining,—in other words, suflicient to bear the polariz- ation. This, though by no means .a complete theory,' is indeed applicable to ultimate atoms, and is the only one which admits decomposition for all electromotive forces. ClauSius shows that the finite electromotive force is necessary to maintain the 1,071.8 in the
free state at the electrodes.
One theory, which we must mention because it accounts at once for conduction, the migration of the ions, and “electric cnd0smose," ls that due to Quincke (Pogg. Amt. , cxiri. . extended in cxliv ), who con- siders the ions of each molecule charged with quantities of electri- city 6 and 6'; then the force K tending to separate the ions from each other = -:—;—: (Be — B'c'), where B and B' are constants, and is the electromotive force per unit of length ot the electrolyte, and is consequently -= - 0%, where i is the current intensity, q the sectional area of the electrolyte, and 15 its specific conductivity; so that K is greater than the force of chemical affinity. This is a weak point of the theory, as a finite electromotive force would be required to produce any decomposition or polarization.
The forces on the ions when separated, and hence their respective velocities, willbe proportional to e and e'. This willaccourit for the migration of the ions, for which 2 and e’ are supposed unequal and of different signs in all cases except Zul and Cdl, &c., for which I; (Be - B'e'), and electrolysis takes place when this 1 . (l - :17) is greater than unity, for these i. and (' may be ot the same sign. If, on the other hand, e be the amount of free electricity on a molecule of the electrolyte (supposed of high resistance) in contact . d . With the glass, then — 13%;: will represent the force urging the fluid in the positive direction of the current, and perhaps producing endosmose, since s will be positive except for turpentine oil. So the motion of particles may be similarly explained by supposing (- to be the charge on them due to contact with the fluid; this is negative with particles in water, and positive for all particles except sulphur in turpentine oil. The results thus obtained will be found to agree closely with the experiments mentioned above (p. 111); and the quantitative results also agree, since the force on a particle i. . . . equals 8—H, and therefore varies as the current densrty z, and inversely as the conductivity 1:.
An application of electrolysis, which has already proved to be of great value in chemistry, has been introduced of late years by Gladstone and Tribe. in a paper read before the British Association in 1872 (Trans. of Sections, p. 75, see Proc, Roy. Soc., vol. xx. p. 218) they showed that although zinc alone does not decompose distilledrwater, yet if zinc forl be immersed in dilute solution of euprie sulphate, and be thereby coated with metallic copper, which is thrown down as a black crystalline powder, containing traces of zinc only if the time of immersion be very long (Journal Chem. Soc., 1873, p. 452), and if the zinc copper couple thus produced be immersed in distilled water at ordinary temperature, about 4 cc. of H can be collected per hour. The hydrogen is seen by the microscope to collect upon the copper crystals, while the zinc is oxidized, and forms a hydrate. The rate of evolution of hydrogen varies with the temperature; the relation may be exhibited by a curve very similar to the curve of tension of water vapour. Gladstone and Tribe have found this a powerful method of acting upon many organic bodies, particularly the halogen compounds of the alcohol radicles. In all cases either new reactions were set up, or the temperature at which reaction takes place was very much lower than with ordinary zinc (see the series of papers by Gladstone and Tribe in the Jour. Chem. Soc, 1873—6). To the chemist the ZnCu couple affords an exceedingly convenient way of arranging electrolysis, since the whole may be contained in one vessel. For the copper in the arrangement, gold or platinum may with great advantage be substituted by immersing zinc foil in solutions of the chlorides.
This easily explains the well-known custom of generating hydro- gen f'rom zinc and sulphuric acid, to which a little CuSO4 is added; and the “local action" in batteries, when currents pass from one part to the other of the same mass of metal and consequently energy is expended for which no external equivalent is obtained, may be similarly referred to the difference of composition of the metals in the two places. It should be remembered that Davy suggested the preservation of the copper sheathing of ships by attaching plates of Zn; the same object is now achieved by usrng an alloy of the two metals.
The application of the principle of the conservation of energy to electrolysis has already produccd_valuable results; research, however, in this direction is rendered difficult on account of the great number of circumstances which have to be taken into account, in computing the balance of energy expended and work done; the chemical composition and physical state of the electrolyte, the molecular condition of the ions, and the secondary actions at the electrodes have all to be taken into account. For a notice of the present state of this branch ot the subject the reader is referred to the article Electricity.
ELECTRO-METALLURGY, a term introduced by the late Mr Alfred Smee to include all processes in which electricity is applied to the working of metals. It is far more appropriate than the French equivalent galvanoplusu'e, or the German Galvanoplasti/c, since the metals are certainly not rendered plastic under galvanic action, though it is true that in electrotypy, which forms one branch of electro- metallurgy, the metal is deposited in moulds, and can thus be used to reproduce works of plastic art.
It was observed as far back as the beginning of the present century that certain metals could be “revived " from solutions of their salts on the passage of a current of electricity. The germ of the art of electro-metallurgy may undoubtedly be traced to the early experiments of ?ollaston, Cruickshank, Brugnatelli, and Davy ; but it re- mained undeveloped until the late Professor Daniell devised that particular form of battery which bears his name, and which hedescribed iii the Philosophical Transact-ions for 1836. A Daniell’s cell consists, in its usual form, of a copper Vessel containing a saturated solution of blue vitriol or sulphate of copper, in which is placed a porous cylinder containing dilute sulphuric acid ; a rod of amalgamated zinc is immersed in the acid, and on the two metals being connected by means of a conductor, electrical action is im- mediately set up. The zinc, which forms the positive or generating element, is dissolved, with formation of sulphate of zinc ; whilst the blue vitriol is reduced, and its copper deposited, in metallic form, upon the surface of the copper containing vessel, which forms the negative or conducting element of the combination. Any one using this form of battery can hardly fail to observe that the copper which is thus deposited takes the exact shape of the surface on which it is thrown down, and indeed presents a faithful counter- part of even the slightest scratch or indentation. Mr De la Rue incidentally called attention to this fact in a paper published in the Philosophical illagazz'ne in 1836, but it does not appear that any practical application was at the time suggested by this observation. Indeed, the earliest notice of electro-metallurgy as an art came from abroad two or three years later.
a letter from Mr Guggsworth, announcing that l’rofessm Jacobi, of St Petersburg, had recently discovered a means of producing copies of engraved eopper- plates by the agency of electricity. This was the first news of the new art which appeared in England, and it evidently referred to the paper which Jacobi communicated to the St Petersburg Academy of Seiences on October 5, 1838, and in which he explained his process. In the Athena-um of May 4, 1839, there was a short paragraph relating to J acobi's discovery, and public attention in this country was thus drawn to the subject. Only four days after the appearance of this paragraph, .\I r Thomas Spencer, of Liverpool, gave notice to the local Polytechnic Society that he would read a paper on a similar discovery of his own. This paper was not read, however, until September 13 3 and although the author wished to describe his process before the British Asso- ciation at Birmingham in August, it appears that. his communication was never brought before the meeting. In Mr Spencer’s paper, which was eventually published by the Liverpool Polytechnic, he states that his attention was first directed to the subject by mere accident: he had used
a copper coin, instead of a plain'piece of copper, in a