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(44.) The essential advantage which this excellent battery possesses over all which preceded it, is the removal of the hydrogen, instead of its evolution at the negative vessel, whilst in the nascent state, by its decomposing the sulphate of copper. It is owing to this also, that this battery gives off no fumes. To employ the decomposing effects of this battery with the greatest advantage, a series of ten or twelve is required.

(45.) Another battery, upon precisely the same principles, although applied in a very different way, was invented by Mr. Grove. He uses, for his negative metal, platinum, and in the inner cell he puts strong nitric acid, and in the outer one, with the zinc, dilute sulphuric or muriatic acid. The form which Mr. Grove prefers, is a many-celled trough like the Wollaston's, with flat paralellopiped porous tubes in the interior; and as platinum is an expensive metal, he takes care that the whole surface is brought into full operation, by completely surrounding it with zinc. In this battery the nitric acid is decomposed by the hydrogen, and deutoxide of nitrogen is evolved; which, coming in contact with the atmospheric air, is converted into nitrous acid.

(46.) This battery is remarkable for its intensity; a series of four being sufficient for most decompositions. A large series exhibits the arc of light in a very brilliant manner; for showing this phenomenon it exceeds all other batteries. This battery however, with its great intensity, is not without some serious disadvantages; for the nitrous fumes which are evolved during its action, are extremely pernicious to the animal economy; so much so, that it might be even dangerous to be exposed to them, without a free access of air. These nitrous fumes will attack almost every metallic surface with which they come in contact; and therefore this battery should not be employed in a room where there are polished stoves or metallic apparatus. The nitric acid moreover passes through the porous tubes, and attacks the zinc to a considerable extent, independently of that zine which is dissolved to generate electricity:—and lastly,

it has the objection of requiring the trouble and expense attending upon porous tubes.

(47.) We have thus seen that Mr. Grove's intense battery is in its principle similar to that proposed by Professor Daniell, for in both the hydrogen is removed by chemical means; in the first instance by nitric acid, and in the second by sulphate of copper. Of course there are many other modes by which the same results may be obtained; as for instance, by using nitrate of silver, or the salts of gold, palladium, and platinum, or by other oxygenated acids, as the iodic, chloric, and bromic. I have tried many other substances upon this principle, but have not arrived at any new result, nor have found any arrangement superior for its intensity to that of Mr. Grove.

(48.) In conducting a series of experiments on the ferrocyanuret of potassium, having had frequent occasion for the use of a galvanic battery, I found, that although the two last were admirably contrived instruments, yet that it was very desirable to possess one that could be set in action at a moment's notice, and with comparatively little trouble. It became thenceforth my endeavour to construct one that should require little or no labour in its employment, and this was followed by the devising of the chemico mechanical battery.

(49.) This battery, after I had minutely investigated every property which belongs to the metals of which batteries are constructed, was made upon noticing the property which rough surfaces possess, of evolving the hydrogen, and smooth surfaces, of favoring its adhesion. Thus whatever metal we use for our negative plate, we take care that it be roughened, either by a corrosive acid, as iron by sulphuric acid, copper and silver by nitric acid, or mechanically by rubbing the surface with sand paper. Even by these means alone the metals are rendered much more efficient; but to take advantage of this principle to the fullest extent, I cover platinum with finely divided black powder of platinum, by galvanic means; that is, I place the platinum as the copper is placed

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in a Daniell's battery; but instead of employing sulphate of copper in the outer vessel, I use a small quantity of nitro-muriate of platinum, so that the finely divided metal is thrown down on the sheet platinum, previously roughened by sand paper. In this way it was also placed on palladium, silver (roughened by nitric acid), plated copper, iron of every sort, and on charcoal, with the same good result; but no other metal was found to answer for its reception. The metal generally employed is silver, because of its cheapness, and its not undergoing any alteration. But whatever metal be used, the principle is the same, viz. the affording a surface to which the hydrogen shall not adhere, but from which it shall be evolved; and the infinity of the points which are presented by such a surface as above described, appears to be the cause of this excellent result. The preparation of the silver is now made a separate branch of a trade, and the platinized metal can now be bought ready for use; but for those who desire to perform this operation, a brief description is here added.

(50.) The metal to be prepared should be of a thickness sufficient to carry the current of electricity, and should be roughened, either by sand paper, as in the case of platinum or palladium, or when silver is employed, by brushing it over with a little strong nitric acid, so that a frosted appearance is obtained. The silver is then washed, and placed in a vessel with dilute sulphuric acid, to which a few drops of nitro-muriate of platinum are added. A porous tube is then placed in this vessel, with a few drops of diluted sulphuric acid; into this the zinc is put. Contact being made, the platinum will in a few seconds be thrown down upon the surface of the silver, as a black metallic powder. The operation is completed now, and the platinized metal ready for use. However, iron, when thus prepared, is as effectual as silver, and may be sometimes employed with advantage. With this metal, all that is required is to rub a little nitro muriate of platinum over it, and an immediate deposit of the black powder takes place.

(51.) The liquid generally adopted to excite this battery, is a mixture of one part, by measure, of sulphuric acid, and seven of water, which will be found amply strong for all purposes. Where we desire greater intensity, we can obtain it by the addition of a few drops of nitric acid, but if too much be used it will attack the silver. When however, platinized platina is employed, the nitric acid may be used with impunity. The electro-metallurgist will find it advisable to use dilute sulphuric acid, containing only 1-16th of the pure acid.

(52.) Numerous enquiries have been made as to what arrangement is best suited to this battery; but this must depend upon the purpose for which it is employed. For the student's laboratory, the Wollaston trough of twelve cells appears to be best adapted (fig. 7 A); and it should be so constructed, that any number of cells may be employed, independently of the others, as they may be required. The silver being the most expensive metal, the zinc shonld completely surround it, so that the whole of the silver may be brought into action. Where a battery is required to continue in action for a very long time, as for days or even weeks, a larger vessel, to contain more dilute acid, must be used (fig 3 A).

(53.) When we desire to employ a battery for manufacturing purposes, it might be as well in some cases to remove the sulphate of zinc as soon as formed, by means of a syphon tube passing to the bottom of the vessel, while fresh acid is continually supplied at the top; but this is not generally necessary. For these purposes the battery should be so constructed, that any of the zinc plates, when worn out, can be readily replaced. There are many other forms which may be adopted; as the circular, with the zinc outside; or it may be used as a tumbler battery.

(54.) The characteristic of this battery is the great quantity of electricity produced, and its simplicity; moreover it requires but very little trouble in its manipulation. The zinc seldom demands amalgamation, as that will generally last till the metal is all dissolved.

(55.) In using this battery it is important that no salt of copper, lead, or other base metal be dropped into the exciting fluid, as by that means the silver would become coated therewith; the plain consequence being, that a surface of copper instead of that of the finely divided platinum is presented to the fluid. From a want of knowledge of this fact, in some who have used the battery, I have seen the negative metal covered with copper, which finally becoming oxidated, rendered the platinum useless.

(56.) Such is a brief view of the three batteries now in use. Professor Daniell's excellent invention being distinguished by its constancy; Mr. Grove's powerful battery, by its intensity; and my own, by the quantity of electricity developed, and by its simplicity. Neither of these can be regarded as a perfect galvanic battery, for each wants some of the properties of the others; it is to be hoped therefore, that every attention will be given to the further improvement of these valuable instruments, until the good properties of each are combined in one. Which of the three is at present to be preferred, must depend upon the purpose for which it is required; and the choice must of course be left to the operator. For my own part it affords me much pleasure to see that the platinized silver battery has fully answered the expectations which I formed of it. By some it has been too much extolled, by others too much blamed. It has been the subject of experiments not appertaining to it, such as comparing its intense effects when quantity is its characteristic. Notwithstanding the mis-statements on both sides, it has fully stood the test of time, and has been employed by the public in a manner which I had not even hoped. The reason they prefer it for general and especially for manufacturing purposes appears to be, that it does not require the use of porous tubes, nor of the strong acids, and that it does not give off poisonous fumes. It will continue in active operation for two, three, or more days, when a sufficiency of acid is supplied to it.

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