« PreviousContinue »
state, as diamond, is as perfect a non-conductor as we know, while in an opaque amorphous state, as graphite or charcoal, it is one of the best conductors: thus in one state it transmits light and stops electricity, in the other it transmits electricity and stops light. It is a circumstance worthy of remark, that the arrangement of molecules which renders a solid body capable of transmitting light is most unfavourable to the transmission of electricity, transparent solids being very imperfect conductors of electricity; so all gases readily transmit light, but are amongst the worst conductors of electricity, if indeed they can be said to conduct it at all. The fact that the molecular structure or arrangement of a body influences, indeed I may say determines, its conducting power, is by no means explained by the theory of a fluid; but if electricity be only a transmission of force or motion, the influence of the molecular state is just what would be expected."
Professor Wheatstone, by fixing metallic points at the extremities of the wires or poles, has found that the appearance of the spectrum of the voltaic arc or vivid flame that is seen between the terminals of a battery, depends, as in static electricity, upon the metal from whence it is taken. The spectrum of that from mercury consists of seven definite rays, separated from each other by dark intervals; these visible rays are two orange lines close together, a bright green line, two blueish-green lines near each other, a very bright purple line, and, lastly, a blue line. It is the same when it passes through carbonic acid gas, oxygen gas, air, or vacuum. The light from zinc, cadmium, tin, bismuth, and lead, in a melted state, gives similar results; but the number, position, and colour of the lines vary so much in each case, and the appearances are so different, that the metals may easily be distinguished from one another by this mode of investigation. The electric spark is considered by M. Angström to be the overlapping of two spectra, one of which belongs to the metal, and the other to the gas through which the spark passes, and that the bright lines vary with the gas as well as with the metal. In an oxygen spectrum the greatest number of bright lines occur in the blue and violet, in nitrogen in the green and yellow, and in hydrogen in the red. These effects must necessarily be connected with the chemical and thermal properties of the gases.
Mr. Grove considers that the colour of the voltaic arc, or flame,
which appears between the poles of a very powerful battery, depends upon the substance of the metal from whence it proceeds and on the medium through which it passes. The spark from zinc is blue, from silver it is green, from iron it is red and scintillating-precisely the colours afforded by these metals in their ordinary combustion. But the colour varies also with the medium through which the light passes, for when the medium is changed a change takes place in the colour, showing an affection of the intervening matter. A portion of the metal terminals or poles is actually transmitted with every electrical or Voltaic discharge, whence Mr. Grove concludes that the electrical discharge arises, at least in part, from an actual repulsion and severance of the electrified matter itself, which flies off at the points of least resistance. He observes that "the phenomena attending the electric spark or Voltaic arc tends to modify considerably our previous idea of the nature of the electric force as a producer of ignition and combustion. The Voltaic arc is perhaps, strictly speaking, neither ignition nor combustion. It is not simply ignition; because the matter of the terminals is not merely brought to a state of incandescence, but is physically separated, and partially transferred from one terminal to another, much of it being dissipated in a vaporous state. It is not combustion; for the phenomena will take place independently of atmospheric air, oxygen gas, or any of the bodies usually called supporters of combustion; combustion being in fact chemical union attended with heat and light. In the Voltaic arc we may have no chemical union, for if the experiment be performed in an exhausted receiver, or in nitrogen, the substance forming the terminals is condensed and precipitated upon the interior of the vessel, in, chemically speaking, an unaltered state. Thus, to take a very striking example, if the Voltaic discharge be taken between zinc terminals in an exhausted receiver, a fine black powder of zinc is deposited on the sides of the receiver; this can be collected, and takes fire readily in air by being touched with a match, or ignited wire, instantly burning into white oxide of zinc. To an ordinary observer the zinc would appear to be burned twice-first in the receiver, where the phenomenon presents all the appearance of combustion, and, secondly, in the real combustion in air. With iron the experiment is equally instructive. Iron is volatilized by the Voltaic arc in nitrogen, or in an exhausted
receiver; and when a scarcely perceptible film has lined the receiver, if it be washed with an acid, it then gives, with ferrocyanide of potassium, the Prussian-blue precipitate. In this case we readily distil iron, a metal by ordinary means fusible only at a very high temperature."
Another strong evidence that the Voltaic discharge consists of the material itself of which the terminals are composed, is the peculiar rotation which is observed in the light when iron is employed, the magnetic character of this metal causing its particles to rotate by the influence of the Voltaic current. In short, Mr. Grove concludes that, although it would be hasty to assert that the electrical disruptive discharge can in no case take place without the terminals being affected, yet he had met with no instance of such a result, provided the discharge had been sufficiently prolonged, and the terminals in such a state as could be expected to render manifest slight changes! *
Some years ago Mr. Grove discovered that the electrical discharge possesses certain phases or fits of an alternate character, forming rings of alternate oxidation and deoxidation on metallic surfaces. A highly polished silver plate in an air-pump was connected with the pole of a powerful inductive battery, while a fine metallic wire, or even a common sewing needle, was fixed at the other pole, and so arranged as to be perpendicular to the silver plate, and very near, but not touching it. By means of this apparatus the electrical discharge could be sent through any kind of rarefied media. In some of the experiments a series of concentric coloured rings of oxide alternating with rings of polished or unoxidated silver were formed on the plate under the point of the needle or wire. When the plate was previously coated with a film of oxide, the oxide was removed in concentric spaces by the discharge, and increased on the alternate ones, showing an alternate positive and negative electricity, or electricity of an opposite character in the same discharge.
When the silver plate was polished the centre of the rings formed on it was yellow-green surrounded by blue-green; then a ring of polished silver, followed by a crimson ring with a slight orange tint on the inner side and deep purple on the outer;
*Correlation of the Physical Forces, by W. R. Grove, Esq.,' one of the most remarkable and talented works that has appeared, to which the author with pleasure acknowledges her obligations.
When the air-pump was rings were precisely the
lastly the indication of a polished one. filled with attenuated olefiant gas the same with those seen in thin plates; hence the effect is the same as that produced by the interference of light. In these experiments the luminous appearance extended from three quarters of an inch to an inch round the point of the needle or wire.
When the silver plate was connected with the negative pole of the battery a polished point appeared upon it opposite the needle, surrounded by a dusky ill-defined areola of a brown colour tinged with purple when viewed in one direction, and greenish-white when seen in another.
In the present year Mr. Gassiot, Vice-President of the Royal Society, has shown that the stratified character of the electric discharge is remarkably developed in the torrecelian vacuum. Among the various experiments made by that gentleman two may be selected as strongly illustrative of this new and singular property of electrical light.
In a closed glass tube about an inch internal diameter and 38 inches long, in which a vacuum had been made, two platinum wires were hermetically sealed, 32 inches apart, and connected with the poles of an inductive battery. The luminous appearance at the two poles was very different when electricity passed through the wires. A glow surrounded the negative pole, and in close approximation to the glow a well-defined dark space appeared, while from the positive pole or wire the light proceeded in a stream; but unless the charge be great or the tube short, the stream will not extend to the black band, which is totally different from the intervening space. When discharges of electricity were sent through this vacuum tube a series of bands or stratifications were formed which were concave towards the positive pole; and as in the changes in making and breaking the circuit the electricity emanates from the different terminals or wires, their concavities were in opposite directions.
When instead of platinum wires narrow tinfoil coatings were placed round the exterior of the glass tube and connected with the wires of the battery, brilliant stratifications filled the interior of the tube between the foil coatings, but no dark band appeared. At present Mr. Gassiot is inclined to believe that the dark band is due to interference; but that the stratifications arise from pulsations or impulses of a force acting in a highly attenuated but
resisting medium, for even with the best air-pumps it is impossible to make a perfect void; he is still occupied with experiments on this new subject, and no doubt will obtain very remarkable results, of which none can be more extraordinary than his discovery of the powerful influence of the magnet on this electric light. The stratifications are formed in rapid succession in the tube with platinum wires and are turned different ways, but they can be separated at any part of the tube by the pole of a magnet round which the whole stratifications have a tendency to revolve. In the second experiment, where the tinfoil was used, the discharge was divided in two by the pole of a magnet, and the two parts had a tendency to rotate round the magnet in opposite directions.
Voltaic electricity is a powerful agent in chemical analysis. When transmitted through conducting fluids, it separates them into their constituent parts, which it conveys in an invisible state through a considerable space or quantity of liquid to the poles, where they come into evidence. Numerous instances might be given, but the decomposition of water is perhaps the most simple and elegant. Suppose a glass tube filled with water, and corked at both ends; if one of the wires of an active Voltaic battery be made to pass through one cork, and the other through the other cork, into the water, so that the extremities of the two wires shall be opposite and about a quarter of an inch asunder, chemical action will immediately take place, and gas will continue to rise from the extremities of both wires till the water has vanished. If an electric spark be then sent through the tube, the water will reappear. By arranging the experiment so as to have the gas given out by each wire separately, it is found that water consists of two volumes of hydrogen and one of oxygen. The hydrogen is given out at the positive wire of the battery, and the oxygen at the negative. The oxides are also decomposed; the oxygen appears at the positive pole, and the metal at the negative. The decomposition of the alkalies and earths by Sir Humphry Davy formed a remarkable era in the history of science. Soda, potass, lime, magnesia, and other substances heretofore considered to be simple bodies incapable of decomposition, were resolved by electric agency into their constituent parts, and proved to be metallic oxides, by that illustrious philosopher. All chemical changes produced by electricity are accomplished on the same principle; and it appears that, in general, combustible