tube containing chloride of silver was found to contain reduced silver: and when potassa had been submitted to the electric current, then the potassium was seen to take fire as it was produced*. 3. ON THE DECOMPOSITION OF METALLIC SALTS BY THE VOLTAIC PILE, AND ON THE STATE OF CHLORIDES, IODIDES, &c. IN SOLUTION. Whilst experimenting with a voltaic pile of thirty pairs of plates, M. Carlo Matteuci observed, that when the poles were plunged into solution of common salt, they both evolved gas; but that when introduced into solution of sulphate of copper, although oxygen was evolved as before from the positive pole, hydrogen ceased to be disengaged at the negative pole, but metallic copper was there deposited. Using various other metallic solutions, he found that those of lead and silver, with some others, produced the same effect, i. e. evolved no hydrogen, but had the metals deposited in the metallic state, whilst others evolved gas at the negative pole, and had their bases deposited as oxides. Reasoning on the effect, he was induced to conclude, that in the cases in question, the hydrogen separated at the negative pole was employed in reducing the oxides of the metals; and hence its disappearance, and the deposition of the base in a metallic state. To assure himself of the truth of this view, he constructed a weak pile composed of only two elements, and incapable of decomposing a weak solution of salt. A solution of nitrate of silver is far more easily decomposed than water, as M. Becquerel has shown, and such a solution was readily decomposed by this weak pile of two elements; and at the same time it was observed, that the usual deposit of metallic silver did not occur, but an olive-coloured layer of oxide of silver was produced. It is, therefore, sufficiently proved, that the disengagement of hydrogen at the negative pole of the pile ceases, only because that element is employed in reducing the metallic oxides already separated from these acids by the action of the pile. It is a striking case of the powers of nascent hydrogen at common temperatures. Having explained this appearance, M. Matteuci proceeded to decompose the chlorides and iodides by means of the pile, with the expectation of being able to deduce the nature of these compounds when dissolved in water. If it were possible to decompose these combinations by means of electric currents, incapable of decomposing water, one might then justly conclude that their composition was not changed by solution in that liquid. He, therefore, took a pile composed of two elements only, charged with water rendered slightly saline, and which had no power of decomposing water even a little acidulated. The platina conductors were then dipped in a solution of muriate of copper, and after some time, the negative conductor Bib. Univ. 1830, p. 213. was covered with metallic copper, whilst the positive conductor evolved bubbles of gas. Having replaced the latter conductor by one of silver, it soon became covered with a yellow film gradually changing to violet, which was considered as chloride of silver. The experiment was repeated with the iodides of zinc and iron; the platina poles had scarcely touched the solutions before the iodine, with its distinctive colour, appeared at the positive pole, and the metals were reduced and deposited upon the negative pole. After these experiments it appears,' says M. Matteuci, that we may affirm with certainty, that these combinations, even when dissolved in water, do not change in their nature, and are not converted, as is often imagined, into muriates, hydriodates, &c. of the oxides of the metals present.** 4. VOLTAIC TEST OF THE STATE OF METALS. It is well known that Dr. Wollaston devised a beautiful little arrangement to ascertain the conducting power of certain crystals having metallic characters, and which ultimately proved to be titanium. If a plate of copper be in contact with a plate of zinc, and part of both plates be immersed in a dilute acid, the copper, by its electric condition, decomposes water and becomes covered with bubbles of hydrogen. If a piece of paper, or a card, be interposed where the two metals were in contact, the copper loses this power altogether, and no bubbles appear on it; but if a small hole be made in the paper or card, and a little piece of metallic matter put there, so as to touch at once both the zinc and copper, then the latter has its full power restored. M. Macaire Prinsep has applied this test more generally; and he found, in the first place, that a metal was necessary to restore the effect-lead, bismuth, tin, &c. reproduced the bubbles; but sulphuret of arsenic, rutilite or oxide of titanium, grey cobalt ore, and the sulphurets of antimony, iron, tin, or lead, produced no effect. Portions of meteoric stone from Aigle and Barbotan, by producing bubbles, showed that they contained uncombined metal; and the method seemed competent to indicate, in all cases, whether the metals used were free, or in a combined condition. As lead gave bubbles, but the sulphuret of lead none, experiments were made with lead, to which sulphur, in increasing proportions, had been added:-10, 30, 3, 6, and of sulphur did not take away the property from lead; but when of sulphur was used, no bubbles appeared upon the copper. Then ascertaining the proportions in the definite sulphuret of lead, he found them to be exactly those which caused the evolution of bubbles to cease (86 lead and 14 sulphur.) The same effect occurred with the sulphuret of tin; and hence it was concluded that chemical combination in *Bib. Univ. 1830, p. 138. determinate proportions was necessary to prevent this electric decomposition, and that mixtures had no influence on the phenomena. These results may be important to the mineralogist; and M. Macaire Prinsep, in illustration, concludes, that the grey cobalt ore of Lunaberg, which is composed of cobalt, arsenic, and sulphur, contains only sulphurets of the metals; that, on the contrary, the metals of a rolites, although sometimes found associated with sulphur, and always with silica, exist neither as sulphurets nor silicates, but in their metallic condition*. 5. POWERFUL ELECTRO-MAGNET CONSTRUCTED BY PROFESSOR MOLL. If a piece of iron rod be bent into the form of a horseshoe magnet, and coiled round with a copper wire, so that the latter may form a helix through which the voltaic current may be sent, the iron becomes for the time a powerful magnet. Professor Moll has repeated this experiment upon an enormous scale. His galvanic apparatus was a copper cell, charged with water, mingled with of sulphuric and of nitric acid, into which was introduced a zinc plate exposing 11 square feet of surface to the acid. His magnet was made of a cylinder of soft English iron, 1 inch in diameter; when bent into form, the interval between the ends was 8 inches: the copper wire forming the spiral was of an inch in diameter, and made eighty-three convolutions; the weight of the whole was 5lbs. A connecting piece, of the usual form, made of soft iron, joined the two extremities of the horseshoe, and the ends of the spiral were dipped in mercury for ready voltaic communication. The horseshoe was hung in the usual manner of magnets. In the first experiment this arrangement sustained, first, 50lbs., and afterwards, with care, 76lbs. by its magnetic attraction. When the suspended weight was small, it was found that the iron retained its magnetism for a time after the voltaic communication was broken. If, instead of merely breaking the direction, the electric poles were reversed, then the reversion of the magnetism took place with extraordinary rapidity. On effecting the change, the iron lost all power, the weight fell off, but, with the rapidity of lightning, was again attracted and sustained to an equal amount as at first. The rapidity of this change is the more extraordinary, if compared with the slowness and difficulty of charging the poles of a magnet, of equal force, by the ordinary method. If, instead of a heavy weight, a light steel needle be in contact with the poles of the electro-magnet, then so rapid is the change that the needle never falls off, for the attractive force is destroyed and re-established before the gravity of the needle has time to remove it sensibly from its first position. When the piece of soft iron connecting the poles is held by the hand during this change, the sensation is of the most extraordinary * Bib. Univ. 1830, p. 146. kind. Powerful attraction is first felt; this on a sudden fails, and the hand with the iron gives way, but the force is so instantly renewed, that the hand is violently drawn up again by an attraction as great as ever. The moment the electric communication is completed, the iron is magnetised to a maximum, and bears its greatest charge. On increasing the voltaic apparatus in force, by adding to it another, exposing 6 square feet of zinc, so as to make 17 square feet of surface altogether, no increase of magnetic power was conferred upon the arrangement; nor by using a higher charge was any increase of power obtained, the maximum effect of the iron had been developed in the first instance. Whether the spiral were of copper or brass wire, made no difference. When it was iron wire, of an inch in diameter, and prevented from touching the curved soft iron by intervening silk, the weight taken up was rather higher, being 861b. A larger soft-iron horseshoe magnet was now made; the iron was 24 inches in diameter; the chord of its arc was 12 inches; the spiral was brass wire of an inch in diameter, and made 44 turns. The magnet weighed 26lbs. and its connecting piece 4lbs. With the voltaic apparatus of 11 square feet, this arrangement supported 139lbs.; this was raised to 154lbs. by using an iron spiral, with silk between it and the magnet. This is the maximum to which M. Moll has carried his experiments; but the force exerted is enormous, and at the same time instantaneous; and it is extraordinary to see an arrangement, which at one moment can support this weight, lose all its force merely by breaking or altering a distant contact and again have it as fully renewed. On trying to heighten the power of an ordinary steel magnet, now capable of supporting 5lbs., but formerly much more, these means failed entirely though left surrounded by the spiral for a long time, its force remained at 5lb. The powerful electro-magnets of soft iron just described have, however, every power of ordinary magnets in touching or affecting steel bars, or in strengthening and reversing the poles of ordinary magnets. There is a magnet in the Teylerian Museum at Harlem, which supports 230lbs.; there are, perhaps, one or two other very powerful ones, but except these, the electro-magnet of Professor Moll is the most powerful of any known magnets, and yet is, probably, far short of what might be effected by similar means*. 6. LAWS OF ELECTRICAL ACCUMULATION. Mr. Harris, of Plymouth, has made an extensive series of experiments on the laws of the accumulation of ordinary electricity. The details of these experiments, with illustrative plates, are published in the Transactions of the Plymouth Institution, 1830. We have not space for more than the conclusions at which he arrives. i. An electrical accumulation may be supposed to proceed by *Bib. Univ., 1830, p. 19. equal increments. A coated surface, charging in any degree short of saturation, receives equal quantities in equal times, all other things remaining the same. The quantity passing from the outer coating is always proportional to the quantity added to the inner. ii. The quantity of matter accumulated may be estimated by the revolutions of the plate of the electrical machine, supposing it in a state of uniform excitation; or it may be measured by the explosions of a jar connected with the outer coatings. It is as the surface multiplied by the interval which the accumulation can pass: when the surface is constant, it is as the interval; when the interval is constant, it is as the surface. It is also as the surface multiplied by the square root of the free action or intensity: when the surface is constant, it is therefore as the square root of the attractive force. iii. The interval which the accumulation can pass is directly proportional to the quantity of matter, and inversely proportional to the surface it is as the quantity divided by the surface: if the matter and surface be either increased or decreased, in the same proportion the interval remains the same. If, as the matter be increased, the surface be decreased, the interval will be as the square of the quantity of matter. iv. The force of the electrical attraction varies in the inverse ratio of the square of the distance between the points of contact of the opposed conductors, supposing the surfaces to be plane and parallel ; or otherwise between two points which fall within the respective hemispheres at a distance equal to one-fifth of the radius, supposing the opposed surfaces to be spherical. v. The free action or intensity is in a direct proportion to the square of the quantity of matter, and in an inverse proportion to the square of the surface: it is directly as the effect of an explosion on a metallic wire, all other things remaining the same. If the matter and the surface increase or decrease together, so in the same proportion the attractive force remains the same. If, as the matter be increased, the surface be decreased, the attractive force is as the fourth power of the quantity of matter. vi. The effect of an electrical explosion on a metallic wire depends exclusively on the quantity of matter, and is not influenced by the intensity or free action. It is diminished by accumulating the matter on a divided surface: it is as the square of the quantity of matter: it is as the square of the interval which the accumulation can pass: it is directly as the attractive force of the free action, all other things remaining, in each case, the same: it is as the momentum with which the explosion pervades the metal**. 7. ON THE EMISSION OF LIGHT DURING THE COMPRESSION of Gases. When certain gases have been suddenly compressed, the evolution VOL. I. * Page 97. FEB, 1831. 2 C |