and other Minerals when exposed to Heat. 37 



being effected with greater difficulty, the recombination requires 

 more time than in the more excitable minerals. To so great an 

 extent does this take place, that though the maximum deviation 

 of the needle, in one instance amounting 115°, took place within 

 a few minutes after the excited mineral was presented to the elec- 

 troscope, in 20 minutes it was hardly diminished, in 40 minutes 

 it was still 95°, in an hour 85° # . After a lapse of several hours it 

 was still considerably excited. I obtained similar results with se- 

 veral crystals. Probably in all minerals the difficulty of decompo- 

 sition and combination increase with the mass ; hence, slender 

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

 Keeping these facts in view, ^Epitsus'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 ^ inch for the 

 side of its cube, I obtained very analogous results, when one of its 

 four resinous poles was presented to the electrometer in a warm 

 state, the disk slowly and regularly receded from zero as the 

 cooling advanced, and in about ten minutes reached its maxi- 

 mum 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 

 20 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 

 facility a high degree of electrical excitement, so much so, that 



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



