ELECTRICITY. 



extremity of each axis. This conjecture he quickly ve- 

 Ib J 



_ 



i """"", .T The electricity of the berate of rnametta, is, in go- 

 ,. , : nrr.il. perceptibly weaker than that of tin- tourmalin ; 

 and in repeating tla- experiments <>f I Liny, particular- 

 ty tniwc M 'hich relate to tin- |i]c> that exercise a repul- 

 **"' : "rce, the small needle, negatively electrified, must be 

 directed to the negative pole; fora tin- repulsive force* 

 re limited to a very small space, the least deviation of 

 tit* Maturely electnliid needle will cause it to l>c at- 

 tracted by the adjacent jvirts of the stone, w hich are 

 nearly in their natural -tate dt' electricity. 



M. Hauy likewise found that the centres of action of 

 the8 poles of this mineral arc situated very near the extre- 

 mities of the axes, and that from these poles the electri- 

 cal dcn.-ity diminishes rapidly, and disappears in other 

 parts of the crystal, a result which he had before obtained 

 Irom the tourmalin. See Hauy Annales de Chimie, tom. 

 ix. p. 59, &c. ; Hauy Ann. du Mia. xv. p. 1.; Hauy 

 Traitc de Mineralogir, tom. ii. p. 342 ; and Traite ae 

 Physique, torn. i. See also Art. 6. of the present Section. 



4. On tlte Electricity o/Mesoli/;>c. 



On the i-lir- This mineral is a species of Keolite, and was found 

 tricity of mo by M . Hauy to possess the property of being excited 

 "W- by electricity. Unfortunately he could not obtain com- 

 plete crystals ; but having detached from its support a 

 crystal about 5^ lines in length, he heated it, and pre- 

 sented it to a silk thread negatively electrified. The 

 thread was instantly attracted by the pyramidal sum- 

 mit of the crystal, and repelled by the fractured part ; 

 hence it follows, that the pyramidal summit possessed 

 negative electricity. Stilbite is not electrical by heat. 

 See Hauy Journal des Mines, No. xiv. p. 8?. ; Hauy 

 Memoires tie I' Institiit. tom. i. p. 54, 55. ; and Many 

 Traite de Mineredogie, tom. iii. p. 159, 160. See also 

 Art. 6. of the present Section. 



5. Oil the Electricity of Calamiite. 



On th dec- This mineral has been long found in the mines of 

 BrL-gaw. It was brought to Paris under the name of 



liLuume. Spath seleniteiLi, and was mistaken for zeolite by seve- 

 ral mineralogists, till M. Pelletier * proved that "it was 

 crystallized calamine. M. Hauy, 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 they 

 are exposed to a fire or to the flame of a candle, and 

 they often retain this polarity several hours after they 

 are cold. In one group of these crystals, sensible in- 

 dications 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 course 

 of a single hour. In another species of calamine, where 

 the crystals were similarly grouped, but presented points 

 of octahedrons, Hauy found the same electrical property. 

 See Hauy, Mem. Acad. Par. 17>S5,'p. 20?.; and Hauy 

 TraiU dc-Mineralogie, vol. iv. p. 164. 



6. On the Relation between the Polarity of Minerals and 

 the Secondary form* of their Crystals. 



We are wholly indebted to M. Hauy for the fine dis- 



"*' ' 



covery that the polarity which minerals receive from p** 

 heat, is related to the ten of their secondary crystals. Elecllu " v - 



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 tiiV'puliriiy" 

 of crystals, however, that become i ' ricalby a change of minerals 

 of temperature, deviate from this symmetry of form ; and the K- 



so that the poles or parts of the crystal where the oppo- r " njlir y 

 ... ' , , ...''. inmu of 



site electricities reside, although they are similarly si- t ) lc i rcrvf . 



tuated at the two extremities of the secondary crystal, tals. 

 yet they differ in their configuration ; " one of them un- 

 dergoing decrements which are evanescent upon the op- 

 jxisite part, or to which decrements correspond that are 

 subjected to another law, a circumstance which may 

 enable an observer to predict before hand, simply from 

 the inspection of the crystal, on what side either specie* 

 of electricity will be found when the crystal shall be 

 submitted to the tc^t of experiment. Thus in the variety 

 of the tourmalin which we shall call isogoxe, and which 

 is represented by Fig. 5. the shape is that of a prism 

 of nine plane siiles. terminated at one end by a summit J..',*]^' 

 having three faces, and at the other by a summit 

 having six faces ; and experiments prove, that the first 

 summit is the seat of resinous electricity, while the se- 

 cond manifests vitreous electricity. 



In the new variety of tojiax. crystals, viz. the oclotex- fieviation 

 decimal aluminous fluateof silica, M. Hauy has recently from ym- 

 detected the same deviation from the rules of symmetry metry m 

 in the secondary form of its crystals. Owing probably to the t P aifa 

 the imperfect state of these crystals, he has not been 

 able to observe the same phenomena in the secondary 

 forms of mesotvpo and calamine. 



But of all the crystals that exhibit this co-relation be- 

 tween the exterior configuration and the electric vir- 

 tue, the most remarkable are those which appertain to 

 an acidulating substance, named borate of magnesia, 

 whose form is generally that of a cube incomplete in 

 all its edges, and farther modified by facets correspond- 

 ing to the solid angles. Here the two electricities 

 act according to the directions of four axes, each of 

 which passes through two opposite solid angles of thc 

 cube, which is the primitive form. In one of the va- 

 rieties, Fig. (i. which we shall call defective, one of the j.-jg. $ m 

 two solid angles situated at thc extremities of the same 

 axis is entire ; the other has given way to a facet 

 s. Now resinous electricity is evinced at the angle, 

 which has not undergone any alteration ; and vitreous 

 electricity 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 pre- Fig. T. 

 ceding which were supplied by the facet, continue to 

 present the same mollification. The other angles situ- Deviations 

 ated similarly to; those which were entire, arc here re- f r >,, , 

 placed each by a like facet s ; but if it existed alone, mctry in 

 the symmetry 'would be found re-established, while the *** bot * 

 law of the phenomenon requires that it should be alter- 

 ed. Therefore, three other facets r, r, r, are observed 

 to be situated about each of the former ; so that the an- 

 gles which they modify, present in this respect a kind 

 of superabundance, in consequence of which this variety 

 has been denominated superabundant borateof magnesia." 

 See Hauy Traite de Physique, vol. i. Ann. du Mas. 

 tom. xvi. p. 1. 



Journal dc Ptytique, Decembrt ITS*. 



