Imatges de pÓgina
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crater. The coldness and impetuosity of the blast, and the quantity of ashes and sand swept by it from the edge of the cone, not to omit a most violent hailstorm, rendered our situation peculiarly unpleasant. The crater was by this time nearly half filled with the scoriæ which had been thrown up from the liquid lava beneath, and floating on its surface had thus accumulated. The surface was funnel-shaped, and from the central depression, the explosions and ejections usually proceeded, the effect of which was, as if a very sudden and violent blast, or disengagement of vapour, rushing up from beneath, carried up with it into the air a quantity of these red-hot masses of scoriæ, raising them to a great height above the crater, as though they had been no heavier than feathers. The appearance of the crater at this time was as an enormous furnace, or a valley of fire, the surface of the scoriæ being nearly black, except at the moments of the explosions, when the whole glowed with the most vivid brightness. The central depression was always at a white heat. Whilst we were sitting on the brink of the crater, a new spot, nearer to us than the old point of discharge, burst forth within the crater with a prodious explosion, the mountain trembling beneath us. A vast quantity of red-hot scoriæ were hurled to an immense elevation, and scattered about on all sides. The violence of the wind, however, drove them from us. As I sat behind the other sons, Lobserved the following curious phenomenon. On the highest parts of the hats, coats, cloaks, hoods, &c. of all those before me, was a phosphorescence, that is to say, a pale greenish light was visible on the most elevated extremities of all parts of their clothing that were exposed, to the wind. A green crape veil, (which I wore to protect my face from the sand and ashes, carried by the wind,) when agitated by the blast, appeared to be edged with a fringe of sparks, resembling the light of a glow-worm. When removed from the influence of the wind, the light ceased; but on allowing it again to float in the gale, the light instantly reappeared. The tips of my gloves on being exposed to the wind bore each a spark, like a glow-worm's tail, and seemed communicable from one finger to the other by contact, not unlike phosphorus. Lower down the mountain, this appearance was not perceivable, though ex*omi borchgrs12

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posed to the same wind. When we left the crater*, the wind and hail were so violent, that it was impossible to keep our törches alight, and we were obliged to grope our way back to the Hermitage in the most complete darkness, over the broken lava, perhaps the most rugged spot on the face of the globe.

ON RADIATING ELECTRICITY, by Charles Bonnycastle, Esq., Professor of Mathematics in the University of Virginia in the United States of America. Communicated by P. BARLOW, Esq., F.R.S.

WHEN BENNET first invented that beautiful little instrument, the gold leaf electrometer, he found that a candle placed upon the top of it greatly increased its sensibility; and other experimentalists were not long in observing that a candle possesses the property of depriving all bodies in its neighbourhood of the electricity imparted to them. About a year ago I made some experiments upon this subject, and arrived at what appeared to me an important fact, that electricity radiate, with great velocity when the body upon whose surface it exists is raised to a white heat. I was much struck with this results from its analogy to what I had before witnessed, when assisting Mr. BARLOW in his researches on the magnetism of hot iron. We had found the magnetic action to cease entirely at the moment when the temperature became that of white heat, or somewhere about 1000 degrees of Fahrenheit; and were at a loss to explain the phenomenon, which I have now little doubt arose from a radiation of the magnetic fluid. A similar analogy occurs in galvanism; the brilliant development of light and heat, which takes place when the connecting wires of a powerful battery are armed with charcoal points, does not commence until the charcoal has become intensely white; but it will afterwards be maintained, although the points are removed several inches asunder, and from all the phenomena, is manifestly a case of radiation.

*It was about half-past twelve o'clock. It occurred to me at the time that it was a powerful discharge of electric fluid; but how excited, or by what laws thus discharged, I could not discover.

Thus, if the view I have taken be correct, it follows that electricity, galvanism, and 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 upon 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. It 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.

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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 the top of gum-lac, and to use the mode of insulation, recommened by SINGEer.

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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, coated plate. The

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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 platė. Vanzo

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1 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

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