On Phospliorescence. g 



The general result of the experiments made upon a very 

 great number of bodies exposed to the light of the sun, 

 leads the author to determine that those which are demi- 

 conductors of the electric fluid are all susceptible of taking 

 fire by these means ; that isolating bodies da not shine 

 equally, some of them doinff so fteljly or with difficulty, 

 and the rest not at all; lastly, that bodies which are con- 

 ductors remain dull : and this happens to the metals, to 

 charcoal, to carburet of iron, to all the sulphurets and me- 

 tallic oxides, with the exception of orpiment, the scmi- 

 vitreous oxides of arsenic and tin, and those of zinc and 

 lead made in the humid way. Among the metallic salts, 

 the author has only found that the muriate of tin, ihe sul- 

 phate and the phosphate of lead, shine after being exposed 

 to the sun. 



A remarkable difference between the inphosphorescence 

 which insulating bodies and good conductors present, either 

 in the light or when subjected to feel)le electrical discharires, 

 while all the dcmi-conductors shine bv these two methods, 

 consists in the facultv which the former have of becoming 

 luminous after very strong discharges ; the latter remain 

 constantlv dull, however strong the explosions may be. 



When bodies have been calcined until they have become 

 inphosphorescent, either upon a healed supporter, by ex- 

 posure to the light, or by a first electrical shock, the two 

 first methods cannot restore their phosphorescence; but 

 they resume it by reiterated discharges, and then they again 

 become equally sensible as formerly to heat, light, and a 

 single electrical discharge. 



The author afterward relates variDtis experiments, which 

 prove that these three modes of phosphorescence increase 

 or diminish by the same circumstances. Thus, the sulphate 

 of soda, subjected to the action of light and to that of elec- 

 tricity in four different states, crystallized, deprived of half 

 its water of crvbtallization, of three-fourths of it, and en- 

 tirely calcined, burnt for the same length of time, whether 

 the phosphorescence was excited by the shock of tlie light, 

 or by that of electricity: viz. in the first instance during 

 6", in the second b", in the third ilOO". in the last 4" only, 

 and with a verv feeble light. Sulphate of potash rendered 

 inphosphorescent^ by ca'cmation, thus deprived of its wa- 

 ter of crystallization, and immediately inclosed in a tube 

 furnished with excilators, resumed after five or six ckcirical 

 discharges the propertv of shining bv isolation, as it did 

 before bein<i calcined: hence we njay conclude, that the 

 /romplcte dtsiccation of phosphorescent gubsiances docs not 



deprive 



