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Thus, if the view I have taken be correct, it follows that electricity, galvanism, und magnetism are radiant fluids ; that they are caused to radiate by heat, and commence at a fixed temperature, which is precisely that at which light itself is first given off in abundance. This law is another instance of the connexion between light, electricity, galvanism, and magnetism, and seems to point out that heat is the universal agent by which repulsion is effected. I have added to the experiments on which it is founded, some remarks npon the theory of electricity when in a state of equilibrium, tending to shew that a condition is requisite which Poisson and Biot have not explicitly mentioned, and which serves as the connecting link by which we pass from the properties of the electric fluid when in a state of quiescence to those it possesses when radiating. I have likewise, with the view of removing a difficulty in electricity, given a practical method of causing the spark to pass in either direction. Though the facts which I have mentioned are all drawn from phenomena that are familiar to the electrician, they appeared to me to have been neglected, or misunderstood, which led to the experiments that I shall now detail.
As it was evident that error might arise from a want of perfect insulation, and from the air getting charged with electricity, I made use of very low charges, and as small and simple an apparatus as possible: at first this consisted only of an electrophorus of one foot diameter, with an insulating stand; a polished gilt button, hung by a single thread of unwrought silk about two feet long; a candle; and a gold leaf electrometer. The last I found it advantageous to construct a little differently from that commonly in use. It was formed of a glass vessel, about three inches high, and two and a half wide: closed at the top with a small plate of talc coated in the middle with tin foil, and from which hung the strips of gold leaf. The sides were not as usual coated with tin foil, but a smaller cylinder, coated in this way, and open at each end, was introduced under the first, and received the electricity from the leaves when they were expanded. See fig. 2.
This arrangement renders the use of the instrument more convenient; whilst the second glass is in its place, the electrometer is as accurate as that commonly used ; and when this glass is removed, the leaves may be made to open permanently, whilst we examine the electricity with which they are charged; without fear of their sticking to the glass, or of their flashing and discharging themselves before the necessary examination has been effected. The plate of talc which forms the top of the instrument, is liable to get slightly excited or charged, and therefore, where great accuracy is required, it is better to have tủe top of gum-lac, and to use the mode of insulation recommened by Singer.
mit Ex. 1. The small plate charged from the electrophorus was held at an inch and a half from the flame of the candle for one minute'; when it was immediately carried to the electrometer, and held near to the coated plate. The whole of the electri. city had disappeared.
Ex. 2. The charged plate was held over a mass of iron weighing several pounds and heated to a dull red. The time as before, one minute; when removed to the electrometer, the plate appeared not to have lost any of its electricity,
Ex. 3. The plate was held over red-hot charcoal, some of the pieces of whch were at a white heat. Lost its electricity.
Ex. 4. As it was conjectured that the small pieces which were white hot might have produced the effect, the plate was held over a similar mass of charcoal, no part of which was above a dull red. No loss.
Ex. 5. The plate was held over various : flames, over the flame of sulphur, of hydrogen, over platinum rendered white hot by a current of hydrogen gas, and finally over the flame of alcohol; the result in every instance, excepting that of the sulphur, was a total loss of electricity. The flame of sulphur deprived the plate of its electricity when it was sufficiently intense to approach whiteness, but when the combustion was confined to a small spot, and the flame was perfectly blue, 'it had little effect in abstracting the electricity of the
i Ex. 6. To determine whether a limit with regard to extent of combustion existed in all bodies as well as sulphur, or whether in the latter a small portion burnt less intensely than a large one, the plate was held over a minute piece of charcoal, a point upon which was kept at a white heat by a blow-pipe. The loss of electricity was total. This experiment is best performed by placing a piece of charcoal about the size of a pea on the plate of the electrometer and hanging the small charged plate over it; whilst the charcoal is nearly extinct no effect is produced, but the instant a brilliant point is produced by the blow-pipe, the leaves of the electrometer diverge, and remain apart: and the whole of the electricity will be found to have left the plate.
Ex. 7th. Robison and others have explained the depriving power of points to a stream of electrified air which they maintain; to discover whether the depriving power of flame might be produced in a similar way, the plate was blown upon by bellows, for four seconds, without any material loss of electricity, which was likewise the result when it was plunged in the vapour of boiling water. From this last experiment we may observe how'small a part of the loss which takes place in all electrical experiments arises from the conducting power of the air, even when charged with moisture; nearly the whole must be attributed to the imperfection of the supports.
It was now sufficiently obvious that combustion, or a temperature sufficient to render most bodies luminous, was requisite to give this power of abstracting electricity at a distance ; whilst the hypothesis of a current of air appeared insufficient to explain the operation by which this abstraction was effected. Before any other could be framed, it became necessary to examine what took place at the light, whilst exerting its abstracting power. After a little reflection, the following experiment was devised for this purpose.
Ex. 8. A candle was insulated by being placed in a ring formed at one end of a wire, which slid through the ball of an insulated pillar ; the other end of the wire terminated in a ball*, and was brought within a quarter of an inch of a similar ball which was connected with the ground; a smaller wire connected the flame of the candle with the ring. About twelve or fourteen inches from the candle and opposite to it was placed the charged electrophorus with the plate down; the arrangement will be understood by a reference to fig. 3. Immediately
One of the wires of Lane's discharger answers perfectly well.
the plate of the electrophorus was raised, sparks were seen to pass between the insulated ball and that which was connected with the ground, and continued to do so at intervals until the plate was nearly discharged. The candle being removed and a sharp point substituted in its place, on repeating the experiment, the abstracting power of the point was found to be insensible at this distance.
Ex. 9. The former experiment was repeated under another form, which admits of examining the nature of electricity produced at, or conveyed to, the light. The disposition of the apparatus is as follows: A metal plate A B, fig. 1, (p. 136) was screwed to a glass tube C D, three feet long, and half an inch diameter. The rod slid through a stand, fourteen or fifteen inches high; and the plate was furnished with a pith ball electrometer E. The gold leaf electrometer, F, with its coated guard, already described, was placed eight inches from the plate, upon an insulating stand. Lastly, a small spirit lamp, of metal, and free from sharp points or edges, was placed upon one of the plates used in Bennet's method of condensing. When every thing was ready, the plate A B was charged until the ball stood at 45°, and immediately afterwards the lamp, by means of its insulating handle, was put on the top of the electrometer. At the instant of contact the leaves of the electrometer flashed and discharged themselves at the guard, and continued to do so for a quarter of a minute. During this time the ball E continued to descend, and at the end of it the plate was completely discharged. Towards the latter part of the experiment, when the motion of the leaves was slow, their electricity was examined by a proof plate, and found to be of the same kind with the electricity abstracted.
These experiments demonstrated that the depriving power of combustion arose either from an actual abstraction of the electricity in the bodies within its influence, or from a radiation of the opposite electricity proceeding from the flame. The results, including even the nature of the electricity in the last experiments, could be explained on either supposition; but the following experiment was more decisive.
Ex. 10. A candle, insulated as in the 8th experiment, was