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ADHESION OF HYDROGEN TO THE NEGATIVE METAL. 11 employed, it still very speedily, in fact, almost instantaneously, loses its power. Now this depends principally, if not entirely, in a single battery, upon the hydrogen's adhering to the negative metal, which thereby becomes coated with a non-conducting surface of hydrogen, and is therefore rendered inoperative. The state of surface influences this adhesive quality.

The reader may readily convince himself of the truth of this. Let him immerse in a tumbler of dilute sulphuric acid, a polished plate of copper, and then place a piece of zinc in contact with the copper below the surface of the fluid. Bubbles of hydrogen will speedily appear upon the surface of the copper, and will soon cover its entire surface. It will be seen that these bubbles, instead of rising to the surface, and escaping as soon as formed (or in other words being evolved) will continue adhering to the metal. This depends upon the principle called heterogenious adhesion, which can only operate when the surface of bodies are brought into very close contact. A smooth surface of metal favours the adhesion of the gas to such an extent, as to counterbalance the force with which it tends upwards to the surface of the fluid. This, considering the difference of specific gravity between hydrogen and water, can by no means be a trifling force. Mechanical roughning by sand paper, obviates in some degree this annoyance, but is by no means entirely a remedy. The mode of overcoming this adhesion will be treated of when we describe my battery. To give an idea of the amount of hydrogen which will adhere to smooth metals, I have frequently seen platinum, the heaviest of all substances, rise, by the force of the hydrogen, to the top of the water, after it had been in contact with zinc.

(34.) The same observations apply to the positive metal; for if even impure zinc be polished, the hydrogen will yet adhere to such an extent, that scarcely any action will take place till the surface is corroded; when it will immediately become violent. There is another mode, however, of over

coming this local action, which has been adverted to in this place, instead of mentioning it before, because I believe its action depends upon the facilitating the adhesion of hydrogen; this mode is the amalgamation of the zinc by mercury. In making a battery, this should never be neglected from its economy, as but a small quantity of mercury is required. It is effected by acting upon the surface of the zinc, either by acid, or by planing the oxidized surface, and then rubbing it with metallic mercury. Let us never forget to whom we owe this discovery, which of itself enables galvanic batteries to be used extensively in the arts. Ages to come will perhaps have to thank the inventor, whom we are too apt to forget because he was neither on the council of the Royal Society, nor a London Professor, yet still the obligation from the public to Mr. Kemp is the same.

The explanation which I have ventured to give of this valuable improvement is the following; the mercury envelopes the small portions of charcoal and foreign metals, and therefore the first gas evolved, adheres so firmly to these, that every foreign point of metal becomes coated, so as to prevent farther action; for of all the metals known, there is none to which the hydrogen sticks so firmly, as to mercury. A very instructive experiment proves that the absence of action depends on the adhesion of the hydrogen; for if mercury with zinc dissolved in it, be placed in dilute sulphuric acid, it will give off no gas, but will be covered with large bubbles; but if a little sulphate of copper, nitrate of silver, or nitro-muriate of platinum be placed in the acid, an instantaneous change ensues, for the hydrogen has not now to be evolved, but is absorbed in the nascent state, to reduce the oxides of these metals.

(35.) In an elementary treatise it is unnecessary to enlarge upon these views, but those desirious of entering into them can consult the Philosophical Magazine for April 1840, or the Transactions of the Society of Arts for that year. An observation of these facts led me to construct the Chemico

mechanical battery, of which we shall speak after we have described the other forms. Before, however, entering upon that subject, there is still another property of metals which has not been adverted to; viz. that the least oxidable metals, as platinum, in common with the metals which have most affinity for oxygen, become coated, or so infilmed with air, that they are rendered useless, because they expose a film of badly conducting substance to the fluid instead of a metallic one. The film may be instantly destroyed by heat, or by strong nitric acid. This fact has been long known, and the familiar experiment of causing iron filings to swim, while magnesia, which is an impalpable powder sinks, is an example. But I believe it had not been noticed, as influencing galvanic effects, till mentioned in the paper before quoted.

(36.) The mode in which the hydrogen is evolved, influences the intensity of the battery; for if removed from the negative metal in the nascent state, by any substance which readily yields oxygen to combine with it, the intensity is greater than when it is evolved. The cause of this is not exactly known; some supposing that it arises from chemical action at both poles of the battery, whilst others explain it by supposing that the hydrogen carries off a certain portion of electricity of tension, as they find that a gold leaf electrometer is affected, when brought near the evolved hydrogen.

(37.) No farther improvement was made in the galvanic battery hitherto described. All previous alterations being as to size or form; as flat cells, round cells; or as to the arrangement of the metals, as to which should be innermost; but these can scarcely be called improvements. At length Professor Daniell turned his attention to the subject, and produced a battery on a principle altogether new.

(38.) The form of battery which he recommended, was from eight inches to two feet in height, and four inches in diameter. The outer vessel is to be made of copper, of which the external part may be painted, as it plays no part in generating electricity; while the inner remains uncoated. Into this cylinder a

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VARIOUS ARRANGEMENTS OF DANIELL S BATTERY.

solution of sulphate of copper is to be poured, instead of the dilute acid used in previous batteries; but now, if a zinc plate were put into this solution, and contact were made, the copper of the solution would be reduced upon the zinc, as well as on the outer cylinder; and thus the battery would cease to have any effect. It therefore became necessary to enclose the zinc in a porous vessel, in order to separate it from the sulphate of copper. This was effected by a piece of the gullet of the ox; and into this, which forms an inner vessel, the zinc, with dilute sulphuric acid, is to be placed. Thus we have an outer copper cylinder with a solution of sulphate of copper, and an inner vessel containing zinc and dilute acid. As soon as contact is made, the zinc is dissolved, and sulphate of zinc is retained in the inner part of the vessel; whilst, instead of the hydrogen being evolved at the negative metal, it reduces the copper from the sulphate of copper. The inner vessel must be looked upon as a disadvantage, because there is no doubt that it lessens the power of the battery. The more porous this vessel is, the greater is the quantity of electricity developed; and so common brown paper, coarse canvass, and porous earthenware tubes are employed, instead of the bladder, or the lining valve of the gullet or intestines, as formerly. Professor Daniell uses for his positive metal cast zinc rods, which he amalgamates; and as a little copper always passes through the porous vessel, this should be repeated every time it is employed. The earthenware tubes immediately after use should be plunged into water, and there kept till all the sulphate of copper is dissolved out; or else, by crystallization it will sometimes disintegrate the vessel.

(39.) Many have thought that the zinc being two inches apart from the copper is too far, and they have used cylinders which approached a great deal closer; but although there is no doubt that by these means increase of power is obtained, yet many more inconveniences attend their application than the employment of the form originally suggested by Professor Daniell. In the use of porous tubes of every sort, whenever

the reduction of a metal takes place, care must be taken that neither of the plates of the battery touch the porous vessel; for otherwise the reduction of the metal will take place upon it, and at length a line of continuity will extend from one to the other. Candidates ever anxious to obtain the fame of a new invention, made the batteries square, oblong, paralellopiped, and even in many other forms, without any real advantage, for all the alterations attended with benefits one way, have counterbalancing disadvantages.

(40.) This battery is principally valuable for its constant effects; that is, for the power which it possesses of generating exactly the same amount of electricity for a long time together; and for this reason it is extremely valuable in many experimental researches.

(41.) To obtain its constant effects however, certain precautions are required; for if the size of the wires used for the communication be altered, or their length either materially increased or diminished, then will the quantity of electricity vary. The distance between the poles, and also their size, must remain the same; and great care must be taken that the porous tubes be of the same texture; for it is to be remembered, that if but one bad earthenware tube be used in a battery of large surface, the quantity of electricity will be influenced throughout.

(42.) Much misunderstanding has arisen from the use of the term constancy; it is often thought to signify long-continued action, whereas these properties are really different; for a battery may be constant, but only remain in action for a short period; and again, a battery might continue in action for years, and not be constant in its action; the property of long continuation however, is by far the more valuable.

(43.) The principal disadvantages of this battery are, first, the length of time required to set it in action; secondly, the trouble and expense attending the use of the porous tubes; and lastly, the necessity of continually re-amalgamating the plates.

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