J 2 On Phosphorescence. 



laminae of about 45° to the axis of the prism, and the edges 

 form the surfaces of it by their superposition : this crystal, 

 when lying on the heated supporter on one of these faces, 

 shines ihrouffhout its whole substance, whatever its thick- 

 ness may be: if we make a section in it parallel to the la- 

 minae, and place this section on the supporter, the crystal 

 remains in phosphorescent. 



Arraiionite takes fire in the same way very well when a 

 crystal of this sulistance rests on the supporter by one of 

 the faces of the prism, and remains constantly obscure 

 when it is the base wb.ich is exposed to the action of the 

 caloric. 



The author tried three sniall diamonds crystallized in 

 octahedrons, and formed, as is well known, of laminae pa- 

 rallel to the faces of this solid : they remained without 

 lights; bi!t upon fracturing one to produce some asperities, 

 it became equally phosphorescent with a cut diamond 

 which SL-rved the author for a term of comparison. Among 

 other dian)onds cut in the same maimer, some easily emitted 

 liilht ; and others remained obscure. Two of them being 

 slighily luminous, the author ascertained with a microscope 

 that the laminae of the one were perpendicular, and those 

 of the other almost parallel to the faces. The first was lu- 

 minous on tiie warm supporter, and the second remained 

 obscure. 



M. Dcssaigncs al?o examined the influence of points and 

 asperities on phosphorescence by insulaiion. The limpid 

 rhoinboidal crystal of Iceland, when exposed to the light, 

 acquires s'cry little phosphorescence from it, while its faces 

 have their natural polish : it becomes luminous when one 

 of its faces is rubbed, and when this face is presented to the 

 light. 



Prisninlic and limpid nrragonile in entire crystals presents 

 but a vtrv feeble ligiit, which disappears almost instantly : 

 but when it i> broken, it becomes very phosphorescent on 

 the faces of its fractures in whatever direction they are 

 made. The apaihite of Werner and the chrysolite of the 

 jewellers present similar phajnomcna, but less marked. 

 Acid phosphate of lime, which the author had crystallized 

 in H mass by slow cooling, w as easily electrified by friction, 

 hut did not shine after having been exposed to the light : 

 on fracturing it in order to destroy the polish of its surface, 

 it became very phosphorescent, but was no longer suscepti- 

 ble of clect'.icity as in the first case: so that the same 

 asperities which c ;mmunicatcd to it the property of shining 

 alter having ijecn exposed to the light, rendered it to a cer- 

 tain 



