approximating them, and by rendering the metallic solution very weak with dilute acid.

(160.) To obtain a crystalline deposit with any given solution, we increase the quantity of electricity, but diminish the intensity derived from the battery. We increase the size of the positive pole, diminish the negative, and we approximate them.

With any given negative plate we can obtain a crystalline deposit, by diminishing the intensity of the battery, enlarging its size, saturating the solution with the salt, and taking care that it is not acid. The positive plate may be enlarged, and approximated to the negative.

With any given battery we can obtain a crystalline deposit, by strengthening the saline solution, and not acidulating it, by diminishing the negative electrode, increasing the positive, and approximating them.

(161.) To obtain the metal in the reguline state, however, is our great object, and to obtain the exact point of evolution of the hydrogen, is important and difficult. With any given solution, if we find the hydrogen too abundant, we may either increase the negative pole, or diminish the positive, which will be sufficient for many cases. But in some cases we are desirous of having the electrodes of the same size, and then we diminish the size of the plates of the battery, or the strength of its charge; for instance, I sometimes use my battery charged with water, and a faint trace of acid. Again, in other cases we are desirous to increase the rapidity of the process, and then the evolution of the hydrogen will be diminished, by increasing the quantity of metallic salt in solution, which will answer every purpose. Variation in the distance between the poles will be found sufficient, in many cases, to regulate the evolution of the hydrogen. The converse of all these properties is equally applicable to those cases where the hydrogen is deficient. By regulating the strength of the metallic solution, and adding more or less dilute acid, the evolution of the hydrogen, in any battery (provided it be sufficiently intense to decompose water) will be perfectly under control.

(162.) Thus, with any amount of salts in solution, with any sized negative plate, with any sized battery, and at all temperatures, we can obtain the reduction of any metal in the state we please. It is true that this excessive refinement has hardly been carried to every metal, yet the principles have been so far accurately demonstrated with such a number of them, as to leave no doubt of their general truth and value. (163.) A pretty experiment was devised to show these laws in a more simple and certain way. Into a strong solution of sulphate of copper, contained in a tall vessel, dilute sulphuric acid, with a little sulphate of copper, was poured gently upon the top; this, being lighter, swam on the top, whilst the saturated solution was at the bottom; a long wire, as a negative metal, was passed to the bottom of the vessel. A circuit was then completed. After a short period the wire was removed, when the bottom was found coated with thick, hard, copper crystals. About the middle it was more flexible, and at the upper part the spongy mass was found. The liquid was then stirred up, in order that an intimate mixture might take place, and another wire subjected to the same experiment, when this was universally coated with copper of uniform texture.

(164.) In detailing the above laws, the battery has been spoken of, because we can, by its means, regulate most exactly the quantity and intensity of the current. The same principles apply to the cases in which the metal on which the reduction is to take place, is made the negative plate, to a piece of zinc, enclosed in a porous tube, but we cannot adapt this with that nicety which the battery admits.

(165.) The quantity of electricity in a single cell apparatus, may be increased by enlarging the zinc plate, by approximating it to the negative, by diminishing, as much as possible, any resistance offered, by the use of the diaphragms, and by adding to the acid of the solution which acts upon the zinc. The quantity of electricity may, in like manner, be diminished, by adopting an opposite course of proceeding. In the use of the single cell apparatus, as in that of the battery, the strength of

the metallic solution to be decomposed, will materially influence the quantity of electricity required for its reduction. The neutrality of the solution, the acidity, or the nature of acidity, will operate in a similar manner. The different conditions have been quite sufficiently adverted to, when speaking of the effects of those circumstances, in the use of the battery. The above facts alone are sufficient to make forcibly apparent the imperfection of the single cell apparatus, and the superiority of the process by the battery.

(166.) There are certain peculiarities appertaining to each metal, and even to each salt of the same metal. Each of these demand somewhat different management, depending upon the circumstances under which the reduction of the metal takes place. The variation in the modes of operating will be entered into in the next chapter.

(167.) And now let us consider the influence which time exerts over these processes. Is it necessary, as all authors have asserted, that the voltaic precipitation should go on slowly? The fundamental laws which regulate the precipitation of metals exclaim at once, by no means. For if the electric power be regulated to the strength of the solution, precipitation may take place at a rapid rate. In fact, we shall hereafter show that the reduction of the metals may be more speedily effected than at first sight appears possible, because the deposition is amenable to the same laws whether it takes place slowly or rapidly.

CHAPTER IV.

Reduction of Gold, 168. Reduction of Platinum, 169. Reduction of Palladium, 170. General Remarks, 171. Reduction of Silver from its Nitrate, 172. From its Sulphate, 173. From its Acetate, 174. From its Hypro-Sulphite, 175. From the Ammonio Nitrate, 176. Reduction of Nickel, 177. Reduction of Copper from its Sulphate, 178. From its Nitrate, 179. From its Muriate, 180. From its Acetate, 181. From its Ammoniuret, 182. Copper Positive Pole, 183. Negative Pole, 184. Reduced Copper, 185. Bronzing, 186. Reduction of Zinc, 187. Reduction of Iron, 188. Reduction of Lead, 189. Reduction of Tin, 190. Conclusion, 191.

(168.) We have now to treat of the metals which may be reduced from their salts by the galvanic current, and of the peculiar management which each salt requires.

The salts of all the noble metals are easily decomposed, and the hydrogen has a great tendency to be evolved during their decomposition; on account of which, particular care must attend the reduction of them all. Gold is generally dissolved in nitro-muriatic acid, forming a chloride or muriate of that metal. This is a very soluble salt, and an excellent conductor. From the quantity of electricity which passes, during its decomposition, the hydrogen appears to have a great tendency to be evolved. The strength of the solution used, may be from the palest in colour, to the most concentrated, taking care to follow the laws already laid down; for if no hydrogen be evolved at the negative pole, the gold will not be precipitated in the least degree, as a black powder; and in that way I have actually worked a solution of gold till it became absolutely colourless. The surface of the negative metal on which it is thrown down, should be bright, and a very small quantity of electricity should pass, no matter what its intensity, in proportion to the strength of the metallic solution. The positive pole should be small, and by varying the size of this, we may regulate, to the greatest nicety, the quantity of electricity passing. By attending to these rules, no difficulty will be found in throwing down gold from the strongest solutions. During the decom

position of this salt, chlorine is freely evolved; and this is apt to attack, slightly, the positive pole, though it will not operate materially upon it. The positive pole should either be of fine platinum or gold wire, and it will be frequently sufficient to place the smallest part of the end in the solution.

The negative pole may be of gold itself, carbon, or platinum, as all these are applicable to the reception of the gold, being unaffected by its solution. Even metals which act upon the solution may be employed, care only being taken that these are not allowed, for a single moment, to be in contact with the fluid, without the galvanic circuit being completed. At first the solution should be very weak, but after a gold surface has been obtained, it may be rendered stronger by the addition of more concentrated solution. The larger the negative plate, the more readily will the reduction be effected; for then the positive pole need not be so extremely small.

There are other soluble salts of gold, as the solution of its oxide in potassa and soda, or their carbonates, and the bromide of gold; but the nitro-muriate will be found amply to suffice for all these purposes. The alkaline solution of gold is less readily decomposed than the nitro-muriate, and for that reason is perhaps preferable; yet the nitro-muriate, with care, will answer every purpose.

(169.) Platinum may exist in solution, as a chloride, muriate, or sulphate. The circumstances which affect the reduction of gold, affect also that of platinum; and the observations already applied to the former, are also applicable to the latter, therefore it is needless here to repeat them. The nitro-muriate of platinum is a ready conductor, and requires but a very small platinum wire for the positive pole. The colour of the platinum thus reduced, is somewhat that of bright steel, forming a strong contrast to the absolute black powder, in which state it is employed for my battery.

(170.) Palladium is soluble in nitric acid, containing a trace of muriatic, and the circumstances to be attended to for its reduction, are the same as those for the preceding metals.