These are gold, silver, platinum, palladium and copper. The other metals which I have reduced in a reguline state, or in the state in which we usually employ them (and not as a black powder, or as crystals) are iron, bismuth, zinc, lead, tin, antimony, nickel, and manganese. Most of these can be used as a solid coating, so as to take the form of any conducting or nonconducting object, or as a thin film to serve as a protection or ornament to the body on which it is deposited. These two purposes are so different, that each will merit a separate book in this treatise; but here, the peculiar management of each metal separately, will be entered into.

(147.) The laws which regulate the deposit of every metal appear to be the same, and although very simple, they have cost me much labour for their development. The properties of which I have here to speak are strictly those which relate to the quality of the metal, which is materially influenced by various circumstances. The reduced metal may be precipitated in three different ways; as a black powder, as a reguline metal, (or in other words a metal having the properties of ductility and malleability) and lastly, it may be thrown down in a crystalline form. Between these there are indeed other intermediate states, or mixtures of two different states, but of which we need not here take any notice.

(148.) Law 1.-The metals are invariably thrown down as a black powder, when the current of electricity is so strong in relation to the strength of the solution, that hydrogen is violently evolved from the negative plate of the decomposition cell. The term violently may seem to require some explanation, for the metal will be thrown down as a spongy powder, even when but few bubbles are given off in the half minute.

(149.) LAW II.-Every metal is thrown down in a crystalline state, when there is no evolution of gas at all from the negative plate, or no tendency to it. When I speak of no tendency to the evolution of the hydrogen, I mean, that either electricity of a much greater tension must pass, or the solu

tion must be rendered of more easy decomposition, before gas would be evolved. The crystals may be small or large according to circumstances to be noticed hereafter.

150. LAW III.-To throw down the metals in the reguline state, we must use a quantity of electricity just sufficient to cause the minutest quantity of gas to appear at the negative plate; in other words, the quantity of electricity passing should cause a great tendency to the evolution of the hydrogen, if it be not actually slightly apparent at the negative metal. To say that hydrogen should be given off slightly from the negative plate, may lead to error unless duly qualified, for it is only meant that a few bubbles should adhere to the negative plate, after the action has been continued for some hours, for there should be no farther evolution of gas. In fact, the reguline state is obtained in the greatest perfection when hydrogen is just at the point of evolution, but yet none is really given off from the negative metal.

151. The causes of these variations are difficult to conceive, but perhaps we may venture a theoretical explanation. We have seen in a former part of the work, that the metals may be reduced in two ways. First, Professor Daniell has shown that the metallic salt yields its base frequently, as the direct result of the passage of the galvanic fluid. Secondly, it has been shown, that the metal may be reduced, as the secondary result, from the agency of the hydrogen, which is derived from the decomposition of water. Now it appears to me possible, that in every case where we require the newly deposited metal in its most flexible form, it is to be obtained principally, or even entirely, from the secondary decomposition, or that by means of the hydrogen. A superabundance causes it to go down as a black powder, but a more moderate quantity ensures the deposition of the flexible metal, or in the state in which it is required for the arts. It is by no means improbable that this flexible state is nothing but an agglomeration of the fine powder, in the same way that rolled platinum is an agglomeration of spongy platinum. I would

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EFFECT OF INTENSITY ON THE DECOMPOSITION.

venture still further to suggest, that the crystallized state may arise from the decomposition of the salt, by the direct action of the electric current.

(152.) Dismissing theories, however, we must remember these facts; that the electric power in any solution, when barely sufficient for the production of hydrogen, causes the reduction of the metal in a malleable and ductile state; that the electric power, when not nearly sufficient to cause the appearance of the gas, throws down the metal in crystals; and lastly, that the black pulverised deposit is produced when there is evolution of the gas.

(153.) A very brief examination of our laws will show that the two properties of galvanic batteries must operate in an important manner in producing these results, and accordingly we find that some circumstances are produced by quantity, others by intensity. The regulation of intensity is, perhaps, of the greatest importance; for on the one hand, economy requires as few cells as possible, and on the other hand, other circumstances require more. Whenever it is possible, the fluid to be decomposed should act on the positive pole of the fluid. Thus, in the decomposition of salts of silver, iron, lead, tin, and copper, we use in the decomposition apparatus, positive poles of these respective metals. This enables us to conduct our precipitations with a single cell, which, with my battery, enables us to obtain any given amount of work with the smallest possible cost. During the decomposition, the metals mentioned above are dissolved precisely to the same amount as that to which the new deposit is obtained. The solution is in the same way, always of the same strength.

(154.) The degree of action of the fluid on the positive poles, or rather of the oxygen and acid transferred to the positive pole, varies with every salt of the same metal. To regulate the action equally in different cases, acids, either of more or less oxygenating power, or in greater or less quantity, are added to the metallic solution to be decomposed. An

increase or decrease of the temperature, influences, materially, the intensity required for different salts, because the current passes at a higher temperature with more facility, as the action on the positive pole is more energetic. These minutiæ have hereafter to be fully discussed; but here I wish to point out, that when possible, only one cell of the battery is to be used, and that where this is rather deficient in intensity, a compensation can be obtained by adding acids to the metallic solution, of more or less affinity for the positive pole, according as that may be required.

(155.) For those cases where we use a positive pole or anode made of platinum, we are compelled to obtain increased intensity by employing a more extensive series of batteries. In these cases, we must use as many cells as will decompose water; three or four in general will be amply sufficient. Beyond the mere capability of decomposing water, I cannot perceive that increment or decrease of the intensity, is of material importance, and the regulation of the quantity must then be made the subject of attention.

(156.) The quantity of electricity passing in any fluid will depend, cæteris paribus, upon the distance between the electrodes, the extent of surface they expose to the fluid, or their relative size one to another. These properties have been already described, when treating of galvanic batteries in general, and therefore do not require farther description in this place. However, a different quantity of electricity is required for every variation in the strength of the solution; as any increase of the metallic salt requires a corresponding increase in the quantity of electricity; and the converse is equally true. The effect, however, of an increase of quantity in any solution where there is not intensity enough to produce much hydrogen, would be only to increase the amount of crystallization, or the size of the crystals.

(157.) The quantity of electricity passing in a solution, curiously influences the state of the crystals, for there are two varieties of this deposit; one of which arises from a deficiency

of quantity, in relation to the strength of the solution, and in this state the new plate of metal is like an aggregation of sand, in fact, like common sandstone, the particles having no more cohesion or consistence. In this state, the plate of metal is in the utmost state of brittleness, and this, we must recollect, is produced by too small a quantity of electricity in a strong metallic solution. The second variety of the crystalline state of metals, arises from a large quantity of electricity, in relation to the size of the plate; thus, by using a very large positive pole, connected with a very large battery of feeble intensity, and by employing, at the same time, a strong solution, large crystals, possessing the utmost degree of hardness, will be thrown down.

(158.) To carry into effect these general laws as we may have occasion for them, let us recapitulate the circumstances which may affect them. First we have the size of the battery, secondly we have the strength of the solution, thirdly we have the arrangement of the poles in the decomposition cell, and finally we have the temperature of the solution to be decomposed.

(159.) In any given solution we may increase the disengagement of the hydrogen to cause a black deposit, by increasing the intensity and quantity of the battery, by a series, by diminishing the size of the negative pole, and enlarging the positive electrode, by approximating the electrodes or poles, and lastly, by increasing the heat. All these conjointly, or any of them separately, will favour the increase of electricity, as they will increase the quantity of hydrogen evolved.

For any given size of the negative plate we can obtain a black deposit, by increasing the intensity and quantity of the battery; by increasing the positive electrode; by diminishing the quantity of metallic salt in the solution and adding to its acid, and by approximating the poles.

With any given battery (provided it will decompose water) we can obtain a black deposit, by diminishing the size of the negative pole, by increasing the size of the positive, by