] 42 Prof. Forbes's Experiments on Tourmaline, tyc. 



besides tourmaline have led me. I have applied Coulomb's 

 electrometer with perfect success to the examination of topaz, 

 boracite, and mesotype, which have all been long known to 

 possess electrical powers. In the case of these minerals, I 

 have been able to extend Becquerel's remarkable law of the 

 intensity of electricity rising to a maximum, when the speed of 

 cooling has become comparatively low, which has not before 

 been demonstrated for any mineral except tourmaline. Topaz 

 possesses the remarkable property of retaining its electricity 

 long after the temperature has ceased to change; probably the 

 decomposition being effected with greater difficulty, the recom- 

 bination requires more time than in the more excitable mine- 

 rals. To so great an extent does this take place, that though 

 the maximum deviation of the needle, in one instance amount- 

 ing to 115°, took place within a few minutes after the excited 

 mineral was presented to the electroscope, in twenty minutes it 

 was hardly diminished, in forty minutes it was still 95°, in 

 an hour 85° # . After a lapse of several hours it was still con- 

 siderably excited ; I obtained similar results with several cry- 

 stals. Probably in all minerals the difficulty of decomposition 

 and combination increases with the mass ; hence, slender cry- 

 stals are most easily excited, and the effect less permanent. 

 Keeping these facts in view, JEpinus's statement that the tour- 

 maline preserves its electricity, when insulated, for several 

 hours, will admit of easy explanation, by supposing that he 

 worked with large and difficultly excitable crystals, similar 

 to this one of topaz, which had at the same time a very high 

 degree of intensity. 



With a large crystal of boracite, having about J inch 

 for the side of its cube, I obtained very analogous results. 

 When one of its four resinous poles was presented to the elec- 

 tromter in a warm state, the disk slowly and regularly receded 

 from zero as the cooling advanced, and in about ten minutes 

 reached its maximum duration, which indicated a high degree 

 of intensity. The diminution of electricity was very slow ; in 

 three quarters of an hour the disk had receded but a little way. 

 A small crystal of boracite being similarly treated, the maximum 

 was speedily gained, and the needle returned to zero in one 

 experiment in twenty minutes, in another in half an hour. The 

 electricity of the disk in these experiments was extremely 

 steady. 



The acicular crystals of mesotype attain with the greatest fa- 

 cility a high degree of electrical excitement, so much so that it 

 required some attention to discover that the maximum inten- 



* The disk during this time was of course slowly parting with its charge. 



