time at which the trap rock of Calton Hill has burst up in an incandescent fluid state. This conjecture, shortly after it occurred to him, was confirmed by the intelligence he received at Kreuznach, in Rhenish Prussia, that the temperature in the porphyry of that locality increases at the rate of from 2° to 3° Reaumur in 100 feet downwards, being more than double or triple the rate of augmentation which had been observed in numerous localities in England, France, and other parts of Europe, in granitic rocks and sedimentary strata, and found to be about 1° Fahr. of elevation of temperature in fifteen yards at the least or in twenty yards at the greatest, as Professor Phillips has shown in his Treatise on Geology, in Lardner's Cyclopædia, from careful observations made by himself and others. The author pointed out, that the mathematical theory of heat,-with data as to absolute conductivities of rocks, such as those supplied by Professor Forbes, and with the assistance of observation on the actual cooling of historic lava streams, such as the great outbreak from Etna which overthrew Catania in 1669, or of those of Vesuvius which may be seen in the incandescent state, and observed for temperature a few weeks or months after the commencement of solidification,-may be applied to give estimates, within determined limits of accuracy, of the absolute dates of eruption of actual volcanic rocks of prehistoric periods of geology, from observations of temperature in bores made into the volcanic rocks themselves and the surrounding strata. 12 T. II. [From Brit. Assoc. Rep., 1855. Part 2. Pogg. Ann., XCIX. 1856.] ART. LXXXVIII. ON THE ELECTRIC QUALITIES OF THE well-known ordinary phenomena of magnetism prove that there is a wonderful difference between the mutual physical relations of the particles of a mass of iron according as it is magnetized or in an unmagnetic condition. Joule's important discovery, that a bar of iron, when longitudinally magnetized, experiences an increase of length, accompanied with such a diminution of its lateral dimensions as to leave its bulk unaltered, is the first of a series by which it may be expected we shall learn that all the physical properties of iron become altered when the metal is magnetized, and that in general those qualities which have relation to definite directions in the substance are differently altered at different inclinations to the direction of magnetization. In the present communication, the author described experiments he had made with assistance in defraying the expenses from the Royal Society, out of the Government grant for scientific investigations—to determine the effects of magnetization on the thermoelectric qualities, and on the electric conductivity, of iron. The first result obtained was, that longitudinally magnetized iron wire, in an electric circuit, differs thermo-electrically in the same direction as antimony from unmagnetized iron. This any one may verify with the greatest ease by applying a spirit-lamp to heat the middle of an iron wire or thin rod of iron a couple of feet long, with a little magnetizing coil of copper wire (excited by a cell or two of any ordinary galvanic battery) adapted to slide freely on it, and so bring a magnetizing force to act on two or three inches in any part of the length of the iron; and, when the ends of the iron conductor are connected with the electrodes of an astatic needle galvanometer of very moderate sensibility, suddenly moving the coil from one side to the other of the flame of the spirit-lamp. The author next explained a series of experiments (not so easily described without the apparatus which was exhibited to the Section, or drawings of it), by which it was ascertained that magnetized iron, with electric currents crossing the lines of magnetization at right angles, differs from unmagnetized iron, thermoelectrically, in the same direction as bismuth, that is, in the opposite way to that previously found for iron magnetized along the line of current; and it was verified that an iron conductor, obliquely magnetized, and placed in a circuit of conducting matter, has a current excited through it when its two polar sides are maintained at different temperatures. The author also described and exhibited an experimental arrangement made, but not yet sufficiently tried, to test whether or not magnetized iron possesses a certain thermoelectric rotatory property which his theory of thermo-electricity in crystalline conductors had led him to believe might possibly exist in every substance possessing, either intrinsically or inductively, such a dipolar directional property as that of magnetism *. Regarding the thermo-electric properties of magnetized steel, the only experiments yet made, being on longitudinal magnetization, showed most decidedly the same kind of effect subsisting with the permanent magnetization, after the magnetizing agency is withdrawn, as had been found in iron while actually sustained in a state of magnetization by the electro-magnetic force. The effects of magnetism on the conductivity of iron both for heat and electricity, in different directions with reference to the direction of magnetization, had been tested by different experimenters with no confirmed indications in the conduction of heat, and with only negative results regarding electric conductivity. The author of the present communication, feeling convinced that only tests of sufficient power are required to demonstrate real effects of magnetization on all physical properties of iron, tried to ascertain the particular nature of the conjectured effect in the case of electric conductivity; and at last, after many unsuccessful attempts, succeeded in establishing, that an iron conductor, sustained in a magnetic condition by a longitudinal magnetizing force, and brittle steel wires retaining longitudinal magnetism, resist the passage of electricity more, or, which is the same, possess less electric conductivity, than the same conductors when * [See foot-notes, of date March 3, 1882, to §§ 163 and 176 of Art. XLVIII. Vol. 1. above, respecting Hall's great discovery; see also Article xcı. below, Part IV. § 161.] unmagnetic. It remains to be seen whether either iron or steel has, when magnetized, the electro-crystalline property of possessing different electric conductivities in different directions; and whether either has the possible rotatory property as regards conduction, which the intrinsically dipolar type of magnetization suggests *. It is important to observe, that both the thermo-electric quality, and the effect on electrical conductivity induced in iron or steel, and sustained by the magnetizing force, are retained with the permanent magnetism in steel after the magnetizing force is removed, as Joule found to be the case with the alteration of dimensions, which he discovered as an effect of magnetism ; while on the other hand, as the author showed in a previous communication to the Section [Art. LXXXVI. above], the thermoelectric quality he had discovered as an effect of mechanical strain, becomes reversed when the constraining force has been removed, if any permanent strain has been produced. * See note, p. 179. ART. LXXXIX. ON THE THERMO-ELECTRIC POSITION OF ALUMINIUM. [Brit. Assoc. Rep., 1855. Part 2.] THE author, through the kindness of Baron Liebig, having been enabled to make experiments on a bar of aluminium with a view to investigating its thermo-electric properties, found that it gave currents when its ends were at different temperatures, and an inch or two of its length was included in the circuit of a galvanometer by means of wires of copper, of lead, of tin, or of platinum, bent round it. These currents were in such directions as to show that the Aluminium lies, in the thermo-electric series, on the side. towards bismuth, of Tin, Lead, Copper, and a certain platinum wire (P2); and, on the side towards Antimony, of another platinum wire (P). They were in the same direction as regards the higher and lower temperatures of the two junctions of the aluminium with the other metal in each case, whether the whole bar was heated so much by a spirit-lamp that it could scarcely be held in the hand, or no part of it was heated above the temperature of the air, and one end cooled by being covered with cotton kept moistened with æther. Taking into account the results of previous experiments which the author had made on a number of different metals, including three specimens of platinum wire (P1, P2, P3), probably differing from one another as to chemical purity, which he used as thermo-electric standards, he concluded that at temperatures of from 10° to 32° Cent., the following order subsists unchanged as regards the thermo-electric properties of the metals mentioned:Bismuth, P., Aluminium, Tin, Lead, P2, Copper, P1, Zinc, Silver, Cadmium, Iron. As he had found that a brass wire, on which he experimented, is neutral to P, at -10° Cent., and to P2 at 38°, P, he infers that at some temperature between -10° and 38° Aluminium must be neutral either to the brass or to P. He intends, as soon as he can procure a few inches of aluminium wire to experiment with, to determine this neutral point, and others which he infers from the experiments already made, will probably be found at some temperature not very low, between Aluminium and Tin, and Aluminium and Lead; and to look for neutral points which may possibly be found between Aluminium and P, and Aluminium and P,, at either high or low temperatures. ვა 3 3 |