6. On the Relation between the Polarity of Minerals and the Secondary forms of their Crystals.
We are wholly indebted to M. Ilauy for the fine dis covery that the polarity which minerals receive from heat, is related to the form of their secondary crystals.
The opposite and corresponding sides of crystals are in general similar, both with respect to the number, the disposition, and the figure of their faces. The forms of crystals, however, that become electrical by a change of temperature, deviate from this symmetry of form ; so that the poles or parts of the crystal where the oppo site electricities reside, although they arc similarly si tuated at the two extremities of the secondary crystal, yet they differ in their configuration : "one of them un dergoing decrements which are evanescent upon the op posite part, or to which decrements correspond that are subjected to another law, a circumstance which may ena ble an observer to predict before hand, simply from the inspection of the crystal, on what side either species of electricity will be found when the crystal shall be sub mitted to the test of experiment. Thus in the variety of the tourmalin which we shall call isogonc, and which is represented by Plate CCXLV. Fig. 5. the shape is that of a prism of nine plane sides, terminated at one end by a summit having three faces, and at the other by a summit having- six faces ; and experiments prove, that the first summit is the scat of resinous electricity, while the second manifests vitreous electricity.
In the new variety of topaz crystals, viz. the octoser.. decimal aluminous fluate of silica, :M. I lauy has recently detected the same deviation from the rules of symmetry in the secondary form of its cry stals. Owing probably to the imperfect state of these crystals, he has not been able to observe the same phenomena in the secondary forms of mesotype and calamine.
But of all the crystals that exhibit this co-relation be tween the exterior configuration and the electlic virtue, the most remarkable are those which appertain to an aci dulating substance, named bora• of magnesia, whose form is generally that of a cube incomplete in all its edges, and farther modified by facets corresponding to I he solid angles. Here the two electricities act accord
ing to the directions of four axes, each of which passes through two opposite solid angles of the cube, which is the primitive form. In one of the varieties, Fig. 6. hich we shall call defective, one of the two solid angles situated at the extremities of the same axis is entire ; the other has given way to a facet s. Now resinous elec tricity is evinced at the angle, which has not undergone any alteration ; and vitreous elects icity at the facet, which supplies the place of the opposite angle; thus making eight electric poles, four for each species of electricity. In another variety. Fig. 7. the solid angles analogous to those of the preceding which were supplied by the facet, continue to present the same modification. The other angles situated similarly to those which were entire, are here replaced each by a like facet s ; but if it existed alone, the symmetry would be found re-established, while the law of the phenomenon requires that it should be altered. Therefore, three other facets r r r, are ob served to be situated about each of the former ; so that the angles which they modify, present in this respect a kind of superabundance, in consequence of which this variety has been denominated superabundant borate of magnesia." See Hauy Traite de Physique, vol. i. dun. du Mus. torn. xvi. p. 1.
On the Electricity produced by the melting of Resinous Bodies.
Mr Stephen Gray was the first person who observed that electricity was produced, when bodies merely chan ged their form. He made his experiments with black and white rosin, stone pitch, shell or gum lac, bees wax, and sulphur. These bodies were compounded in diffe rent ways, so as to form the substances in the following Table : the first column of which contains the name of the substance, either simple or compound; and the se cond its weight.