210 



UNDULATORY FORCES. ELECTRO-METALLURGY. [LAWS or DM-ORITI .N 



with acids, there exist thermal nx well as electrical and 

 .iical conditions <if deposition. In a series of ex- 

 perimenta with two plates of antimony immersed in a 

 lid, the two plates being maiutained at 

 different temperatures, and ntiinonius liquids being tried, 

 a weak current ty was developed, wliicli pasted 



from tho hot metal through the liquid to the cold one, 

 the hot metal thus being positive. This occurred in all 

 canes except with liquids, containing uncombiued nitric 

 acid. 



47. Inflitsnix of Temperature on fkt De/toriting Liquid. 

 Tho strength of affinity between tho different element* 

 of a liquid undergoing electrolysis, varies with tho 

 temperature of the liquid ; l>cing almost invariably 

 diminished by elevation of temperature. Rise of tem- 

 perature increases the electric conductibility of an elec- 

 trolyte, and decreases that of tho metal plates immersed 

 in it ; but tho decrease of conductibility of the latter is 

 small in proportion, at moderate elevations of tempera- 

 ture, compared with tho increase of the former ; conse- 

 quently, the general effect of heating a depositing liquid, 

 i.i t<> increase the rapidity of deposition. 



48. Wo have repeatedly observed, that with some solu- 

 tions used at a high temperature for depositing, tho 

 cathode being immersed in tho liquid at the ordinary at- 

 mospheric temperature, and the liquid then heated to 

 tho desire*! poiut, no conduction or deposition took place ; 

 nor did it occur when tho receiving metal was taken 

 out, washed in cold water, and re-immersed ; but if the 

 temperature of the liquid were first raised, and then the 



eatliode suddenly immersed, deposition took place 

 lively, and the liquid might be cooled down many 

 degrees without stopping the action. In coating iron 

 i.'li tin in some solutions, tho iron being immersed 

 before heating the liquid, no deposition took place even 

 at 100 Fall. ; but if tho liquid were first heated, deposi- 

 tion occurred below 100 Fall. 



49. Influence of Liyht upon Deposition. Light appears 

 to exercise much less influence upon electro-deposition 

 than he.it : in some cases, however, where the elements 

 of a depositing liquid are held together by unstable 

 affinities, it decomposes the liquid, and renders it unfit 

 for deposition. For instance, a solution formed by dis- 

 solving hyposulphato of silver in a solution of hyposul- 

 phite of soda, has a tendency to be decomposed in this 

 way. 



50. Dynamic or Mechanical Conditions. The various 

 phenomena occurring in a liquid undergoing electrolysis, 

 may bo viewed, in a mechanical or dynamic aspect, as a 

 series of ininuto movements (attractions and repulsions) 

 occurring between the various particles of matter com- 

 IKNiing the opposed surfaces of tho liquids and of tho 



:.s immersed in thorn. For instance, if we immerse a 

 piece of zino in a solution of sulphate, of copper, and 

 connect it with some mercury in tho same liquid, by a 

 pUtina wire protected from tho solution by a tube of 

 glass or gutta-percha, the particles composing the surface 

 of th ng all electro-positive, will tend to i 



each other, and tho particles of tho liquid surface in 

 contact with it, lieing rendered electro-negative, will also 

 t<-nd t repel e:i liilst tho particle* of acid near, 



Ixjim/ electro-negative, will attract the particles of zinc, 

 which are electro-positive, and tho two will combine 

 together and form a salt ; at tho same time, tho particles 

 if tho liquid surface in contact with tho mercury, being 

 mode electro- positive, will tend to repel each other. 

 Tho particles of the opposed mercury surface, !> 'in;,- 

 lender.-d electro-negative, will also tend to repel each 

 other; whilst tho particles of tho mercury, being electro- 

 negative, and the contiguous particles of copper in the 

 liquid being electro-positive, the two will combine toge- 

 ther and form an alloy. Tho deposition of copper upon 

 , in this case, must bo wholly disregarded, lie- 

 cause it is quito a separate and distinct phenomenon. 



To put this in a clearer form suppose (Fig. 7'- 1 ) thn 

 vortical row of particles Cu, Cu, Cu, Cu, to re 

 the copper anode of a sulphate of copper depositing 

 liquid, and the row of particles Hg, Hg, Hg, Hg, to be 

 the mercury cathode, Pt, Pt being tho connecting wires 



from tho battery, and the double row represent 

 particles of acid SO', and copper On, composing the in- 



Kig. It. 



tervening liquid. Tho particles of t!io nnoii. 

 being all electro-positive, tend to rcpe] each other; and 

 the contiguous particles of Cu and S<>', being all i 

 tivo, also tend to repel each other ; whilst the part idea of 

 tho copper an< 



particles of negative ncid, and romliiiic with them, and 

 form a salt. At tho same time, the particles of Cu and 

 SO' nearest the mercury, being all positive, tend to repel 

 each other ; whilst tho contiguous particles of mercury 

 being negative, and the copper in the liquid being posi- 

 tive, attract and combine with each other and form an 

 alloy. J!y this combination of simultaneous movements, 

 the copper anode dissolves, tho mercury (or any other 

 conducting substance which forms tho cathode) receives 

 a deposit, and the particles of copper of the liquid are 

 gradually removed and replaced by those from the 

 anode. 



61. These attractions and repulsions, like ordinary 

 chemical actions, are all supposed to take place at in- 

 sensible distances, at the mutually opposed surfaces of 

 the liquids and metals, and not to extend into their 

 masses, except so far as they are mixed with each 

 other by capillary attraction or ordinary n. 

 motion, and can therefore only take place where one or 

 both of the substances are in a liquid state ; if it were 

 otherwise, the combinations and decompositions of tho 

 whole masses would probably occur instantaneously. 



62. In addition to those minute and invisible move- 

 ments of the particles, there are other and somet 

 visible movements produced by capillary attraction and 

 by difference of specific gravity in tho liquids. For in- 

 stance, tho salt formed at the anode, if it be soluble in the 

 liquid, will be dissolved and gradually diffused tin. 



it by capillary attraction or adhesion ; whilst fron. 

 greater specific gravity than the remainder of the liquid 

 when dissolved, it tends to sink towards the bottom ; at 

 the same time, tho acid set free at the cathode is like- 

 wise gradually diffused through the liquid by similar 

 means, and from its less specific gravity tends to rise to 

 tho surface. These movements are of general occur- 

 rence during deposition ; and in some solutions, especially 

 if they are very dense and possess a colour, 

 plainly visible to tho unassisted eyo. Their occurrence 

 explains why the substances set free by deposition 

 not instantaneously transferred from ono electrode to 

 the other, but occupy, especially in dense liquids and 

 electrodes far asunder, a considerable period of time in 

 their transference; it also explains why, if depositing 

 solutions are not occasionally stirred, their upper portions 

 become exhausted of metal, whilst their lower parts 

 become deficient of ncid. Motion of the cathode is 

 generally considered necessary to make the deposited 

 metal harder. 



;">.'!. I'lixitlon <>f ihf. Klrctrodes. Tho position of tho 

 electrodes has a considerable, influence upon the pho- 

 nomena of electro-deposition. For in--tance 1st. If tho 

 two electrodes in a depositing liquid are horizontal, with 

 the anode above and the cathode below, tho salt funned 

 at tho anode will, by virtue of its greater specific gravity, 

 sink in tho liquid ; whilst tho acid set free at the cathode 

 will, by its less specific gravity, rise upwards, and thus 

 tho anode will bo constantly supplied with fresh uncom- 



