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friction; for Sir David Brewster observed that bodies deprived of the faculty of emitting the one are still capable of giving out the other. Among the bodies which generally become phosphorescent when exposed to heat, there are some specimens which do not possess this property; wherefore phosphorescence cannot be regarded as an essential character of the minerals possessing it. Sulphuret of calcium, known as Canton's phosphorus, and the sulphuret of barium, or Bologna stone, possess the phosphorescent property in an eminent degree; and M. Edmond Becquerel has shown that, on these substances, a very remarkable phosphorescent effect is produced by the action of the different rays of the solar spectrum. In former times Beccaria stated that the violet ray was the most energetic, and the red ray the least so, in exciting phosphoric light. M. Becquerel has shown that two luminous bands, separated by a dark one, are excited by the solar spectrum on paper covered with a solution of gum-arabic, and strewed with powdered sulphuret of calcium. One of the luminous bands occupies the space under the least refrangible violet rays, and the other that beyond the lavender rays, so that the dark band lies on the part under the extreme violet and lavender rays. When the action of the spectral light is continued, the whole surface beyond the least refrangible violet shines, the luminous bands already mentioned brightest; but all the space, from the least refrangible violet to the extreme red, remains dark. If the surface, prepared with either the sulphuret of calcium or the Bologna stone, be exposed to the sun's light for a short time, it becomes luminous all over; but when, in this state, a solar spectrum is thrown upon it, the whole remains luminous except the part from the least refrangible violet to the extreme red, on which space the light is extinguished, and, when the temperature of this surface is raised by a lamp, the bright parts become more luminous and the dark parts remain dark. Glass stained by the protoxide of copper, which transmits only the red and orange rays,

together with the chemical rays that accompany them, has the same effect with the less refrangible part of the spectrum; hence there can be no doubt that the most refrangible and obscure rays of the spectrum excite phosphorescence, while all the less refrangible rays of light and heat extinguish it. It appears, from the experiments of MM. Biot and Becquerel, that electrical disturbance produces these phosphorescent effects. There is thus a mysterious connexion between the most refrangible rays and electricity which the experiments of M. E. Becquerel confirm, showing that electricity is developed during chemical action by the violet rays, that it is very feebly developed by the blue and indigo, but that none is excited by the less refrangible part of the spectrum.

Paper prepared with the sulphuret of barium, when under the solar spectrum, shows only one space of maximum luminous intensity, and the destroying rays are the same as in sulphuret of calcium.

Thus the obscure rays beyond the extreme violet possess the property of producing light, while the luminous rays have the power of extinguishing it.

The phosphoric spectrum has inactive lines which coincide with those in the luminous and chemical spectra, at least, as far as it extends; but, in order to be seen, the spectrum must be received for a few seconds upon the prepared surface through an aperture in a dark room, then the aperture must be closed, and the temperature of the surface raised two or three hundred degrees; the phosphorescent parts then shine brilliantly, and the dark lines appear black.

Since the parts of similar refrangibility in the different spectra are traversed by the same dark lines, rays of the same refrangibility are probably absorbed at the same time by the different media through which they pass. Multitudes of fish are endowed with the power of emitting light at pleasure, no doubt to enable them to pursue their prey at depths where the sunbeams cannot penetrate. Flashes of light are fre

quently seen to dart along a shoal of herrings or pilchards; and the Medusa tribes are noted for their phosphorescent brilliancy, many of which are extremely small, and so numerous as to make the wake of a vessel look like a stream of silver. Nevertheless, the luminous appearance which is frequently observed in the sea during the summer months cannot always be attributed to marine animalculæ, as the following narrative will show:

Captain Bonnycastle, coming up the Gulf of St. Lawrence on the 7th of September, 1826, was roused by the mate of the vessel in great alarm from an unusual appearance. It was a starlight night, when suddenly the sky became overcast in the direction of the high land of Cornwallis country, and an instantaneous and intensely vivid light, resembling the aurora, shot out of the hitherto gloomy and dark sea on the lee bow, which was so brilliant that it lighted everything distinctly even to the mast-head. The light spread over the whole sea between the two shores, and the waves, which before had been tranquil, now began to be agitated. Captain Bonnycastle describes the scene as that of a blazing sheet of awful and most brilliant light. A long and vivid line of light, superior in brightness to the parts of the sea not immediately near the vessel, showed the base of the high, frowning, and dark land abreast; the sky became lowering and more intensely obscure. Long tortuous lines of light showed immense numbers of very large fish darting about as if in consternation. The spritsail-yard and mizen-boom were lighted by the glare, as if gas lights had been burning directly below them; and until just before daybreak, at four o'clock, the most minute objects were distinctly visible. Day broke very slowly, and the sun rose of a fiery and threatening aspect. Rain followed. Captain Bonnycastle caused a bucket of this fiery water to be drawn up; it was one mass of light when stirred by the hand, and not in sparks as usual, but in actual coruscations. A portion of the water preserved its luminosity for seven nights. On the third

night, the scintillations of the sea reappeared; this evening the sun went down very singularly, exhibiting in its descent a double sun; and, when only a few degrees high, its spherical figure changed into that of a long cylinder, which reached the horizon. In the night the sea became nearly as luminous as before, but on the fifth night the appearance entirely ceased. Captain Bonnycastle does not think it proceeded from animalculæ, but imagines it might be some compound of phosphorus, suddenly evolved and disposed over the surface of the sea; perhaps from the exuviæ or secretions of fish connected with the oceanic salts, muriate of soda, and sulphate of magnesia.

The aurora borealis is decidedly an electrical phenomenon, which takes place in the highest regions of the atmosphere, since it is visible at the same time from places very far distant from each other. It is somehow connected with the magnetic poles of the earth, and occasions vibrations in the magnetic needle. M. Arago has frequently remarked that the needle was powerfully agitated at Paris by an aurora that was below the horizon, and consequently invisible, but whose existence was known from the observations of the polar navigators. The aurora has never been seen so far north as the pole of the earth's rotation, nor does it extend to low latitudes. It generally appears in the form of a luminous arch, stretching more or less from east to west, but never from north to south, the most elevated point being always in the magnetic meridian of the place of the observer; and across the arch the coruscations are rapid, vivid, and of various colours, but whether there be any sound is still a disputed point. A similar phenomenon occurs in the high latitudes of the southern hemisphere. Dr. Faraday conjectures that the electric equilibrium of the earth is restored by the aurora conveying the electricity from the poles to the equator.

SECTION XXIX.

Voltaic Electricity-The Voltaic Battery-Intensity-Quantity-Comparison of the Electricity of Tension with Electricity in Motion-Luminous Effects -Decomposition of Water-Formation of Crystals by Voltaic ElectricityElectrical Fish.

VOLTAIC electricity is of that peculiar kind which is elicited by the force of chemical action. It is connected with one of the most brilliant periods of British science, from the splendid discoveries to which it led Sir Humphry Davy; and it has acquired additional interest since the discovery of the reciprocal action of Voltaic and magnetic currents, which has proved that magnetism is only an effect of electricity, and that it has no existence as a distinct or separate principle. Consequently Voltaic electricity, as immediately connected with the theory of the earth and planets, forms a part of the physical account of their nature.

In 1790, while Galvani, Professor of Anatomy in Bologna, was making experiments on electricity, he was surprised to see convulsive motions in the limbs of a dead frog accidentally lying near the machine during an electrical discharge. Though a similar action had been noticed long before his time, he was so much struck with this singular phenomenon, that he examined all the circumstances carefully, and at length found that convulsions take place when the nerve and muscle of a frog are connected by a metallic conductor. This excited the attention of all Europe; and it was not long before Professor Volta of Pavia showed that the mere contact of different bodies is sufficient to disturb electrical equilibrium, and that a current of electricity flows in one direction through a circuit of three conducting substances. From this he was led, by acute reasoning and experiment, to the construction

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