3. On the Electricity of the Borate of Magnesia.
In the year 1791, M. Hauy obtained two crystals of that species of the boracite or borate of magnesia, which he calls defective, (Magncsie Boratee Defective; see Our article CRYSTALLOGRAPHY, vol. vii. p. 315. col. 2.) He exposed them to the heat of the fire, and presented them to an electrometer.* Ile immediately perceived that it was electrified, and that it had several poles en dued with opposite electricities. He experienced at first considerable difficulty in determining the precise po sition of these poles, both from the delicacy of the ex periment, and from the extreme smallness of the crystals. By comparing, however, the crystals with the tourma lins, he reasoned in the following manner : As in the case of the tourmalin, there was only one axis which coincided with that of the nucleus, there ought only to be two electrical poles situated at the extremity of this axis. In the cube, on the contrary, which was the nu cleus of borate of magnesia, there were four axes, each of which passed through the solid angles, and conse quently there ought to be 8 electrical poles, one at the extremity of each axis. This conjecture he quickly verified by experiment.
The electricity of the borate of magnesia, is, in ge neral, perceptibly weaker than that of the tourmalin ; and in repeating the experiments of Hany, particular ly those which relate to he poles that exercise a repul sive force, the small needle, negatively electrified, must be directed to the negative pole; for as the repulsive for ces are limited to a very small space, the least deviation of the negatively electrified needle will cause it to be at tracted by the adjacent parts of the stone, which are nearly in their natural state of electricity.
M. Hauy likewise found that the centres of action of the 8 poles of this mineral are situated very near the extremities of the axes, and that from these poles the electrical density diminishes rapidly, and disappears in other parts of the crystal, a result which he had before obtained from the tourmalin. See Hauy Annales de Chimie, torn. ix. p. 59, &c. ; Hauy Ann. du Mus. xv. p.
1.; Hauy Traite de Mineralogie, torn. ii. p. 342; and Traite de Physique, torn. i. See also Art. 6. of the pre sent Section.
4. On the Electricity of Mesotype.
This mineral is a species of zeolite, and was found by M. Hauy to possess the property of being excited by electricity. Unfortunately he could not obtain complete crystals ; but having detached from its support a crystal about 5?,- lines in length, he heated it, and presented it to a silk thread negatively electrified. The thread was instant ly attracted by the pyramidal summit of the crystal, and repelled by the fractured part ; hence it follows, that the pyramidal summit possessed negative electricity. Still bite is not electrical by heat. See Hauy Journal des Mines, No. xiv. p. 87.; Hauy Memoires de l' Institut. tom. i. p. 54, 55.; and Hauy Traite de 11•Iineralogie, torn.
iii. p. 159, 160. See also Art. 6. of the present Sec tion.
5. On the Electricity of Calamine.
This mineral has been long found in the mines of Brisgaw. It was brought to Paris under the name of Spath selenitcux, and was mistaken for zeolite by seve ral mineralogists, till M. Pelletier* proved that it was crystallized calamine. M. Ilauy, so early as the year 1785, discovered the electrical property of this mineral, which is the more remarkable as it belongs to the class of metallic substances.
The crystals of calamine are so easily excited, that their polarity appears in two or three seconds after the) arc exposed to a fire or to the flame of a candle, and they often retain this polarity several hours alter they are cold. In one group of these crystals, sensible indi cations of electricity were distinctly visible twelve hours after they were cold, while a tourmalin, excited at the same time, had lost all its polarity in the cow se of a single hour. In another species of calamine, where the crystals were similarly grouped, but presented points of octahedrons, Ilauy found the same electrical property. See Hauy, Mem. Acad. Par. 1785, p. 207. ; and Hauy Traite de Mineralogie, vol. iv. p. 164.