4 On Phosphorescence. 



liquid stale. The third kind of phosphorescence^ which 

 may be, generally speaking, cdWfd phosphorescence by col- 

 lision., supposes a texture ot a gravelly rather than of a glassy 

 nature, and we do not obtain it at its highest degree, ex- 

 cept by the contact of a body of one and the same species, 

 or of a still harder body. It also supposes the hardness 

 of the constituent molecules, but it is not necessary that 

 thev should be strongly united together: thus, sugar, in 

 which the molecides feebly adhere, is, as we know, very 

 phosphorescent upon collision. 



Spontaneous phosphorescence is presented during certain 

 combinations wherein the molecular action is energetic, like 

 that which combines lime and water, the phosphorescence 

 lasts but a few seconds, and the slower the combination 

 lakes place, it is the more nermanent: this happened in the 

 case of all tl-e v>?oods which the author subnntted to experi-^ 

 niei'.t, whether they were growing or cut down. They be-i 

 come equally shining, in both cases when they are pene- 

 trated with humidity, in contact with the atmospheric air, 

 and at a temperature of 8 or 10° of the centigrade thermo- 

 meter. 



We shall now give an idea of the principal facts ascer- 

 tamed by the author, relative to these four kinds of phos- 

 phorescence. So far from the phosphoric light being re- 

 garded as a consequence of the incandescence of the sup- 

 porter, it disappears completely in the case of various bodies, 

 which shine very well on a support heated merely to 200". 

 It results I'rom the experiments of M. Dcssaignes.that bodies 

 which are phosphorescent at an elevation of temperature 

 are equally so on metallic supporters, on those which are 

 had conductors of caloric, like glass or porcelain, and in 

 boiling ntercury or even in water, when substances are 

 acted upon which require a temperature of 100'^ only to 

 shine. The fluate of lime, the phosphate of lime of Estre- 

 niadura, and the adular stone in powder, shine at 100° or 

 112^ of the centigrade thermometer : glass, sand, porcelain, 

 and in general all the vitreous stones, do not shine com- 

 pletely except at 375°. All bodies which are phosphore- 

 scent at an elevated temperature require middle degrees of 

 heat between these two extremes: the whole shine more or 

 less at 256% a tentperature determined by the fusion of bis- 

 nu'.ih. 



It results from many experiments that the light which 

 escapes is in a direct ratio with the degree of temperature, 

 i\\M\ its duration in an inverse ratio. A $ui)Slance which, 

 ).kc filiate of lime, shine? v/ell at a low temp; r;'.tiire, ceases 



t9 



