THE 



PIJILOSOPinCAL MAGAZINE. 



I. Extract from a Memoir on Fhosphorescence, pretenlpd 

 to the Insiitulc, by M. Dessaignks*,o« ^pril 5, ISO9. 



JL HE above memoir is divided into Bve chapters. In the 

 first, the author treats of phosphijrei^cence in general, and 

 of its various modes. He defines phosphorescence to be 

 '*' an appearance ot" light, durable or fuiiitive, not sensibly 

 coniaining any heat, and without any subsequent alleraiion 

 in inorganic bodies." He distinguishes four kinds of phos- 

 phorescence, viz. Fhosphorescenceby the temperature being 

 raised, that which bodies exhibit when exposed to the light 

 or to electrical discharges, that which arises from percussion, 

 from pressure, or from friction, and last of all spontaneous 

 phosphorescence. 



The author recognised the first of these various modes 

 of phosphores,c,ence in almost all bodies presented to us by 

 nature, with but very few exceptions. For this purpose 

 he placed them on a metallic stalk in a dull heat, in order 

 that the lustre of a red hot support might not hinder him 

 from observing the phosphoric light in bodies in whicli it 

 shines but feebly. He made his experiments on bodies 

 which light renders phosphorescent in a dark room, fur- 

 nished at the side exposed to the sun with a small trap- 

 door which opened and shut at pleasure. Exposure to the 

 rays of the sun for a few seconds is surticient for exhibiting 

 the shining of these bodies, the instant we intercept them, 

 with all the brilliancy of which they are susceptible. The 

 light which they give out in this case, is, in general, in- an 

 inverse ratio to their degree of humidity : but they do not 

 lose entirely this kind ui' phosphorescence, except when 

 they are mixed with a quantity of water sufficient for hold- 

 ing them in solution, or when they themselves pass to the 



* Jiiiirjial dct Mines, torn, xxvii. p. '?.\?>. 

 Vol. 38. No. 159. J///y 1811. A2 liquid 



