MEMOIR VIII.] TIIK PHOSPHORESCENCE OF BODIES. 



of diamond, that when it glows it undergoes a kind of 

 surface combustion, which is the cause of the light ; but 

 though direct experiments prove that this is not the 

 case, it is much better to resort to fluor-spar, which is 

 free from such an objection. It is absolutely incombus- 

 tible. Besides, it can be obtained perfectly transparent 

 or nearly opaque ; it occurs of many tints of color ; can 

 be easily cut and polished to any figure, and obtained in 

 pieces of any required size. Its phosphorescent powers 

 are very high ; indeed, it yields, when properly treated, 

 to no other substance, not even to Canton's phosphorus, 

 in that respect, and greatly exceeds the potash sulphate, 

 a substance which, however, possesses many eligible 

 qualities. It will therefore be understood that in select- 

 ing fluor-spar and its varieties as the subjects for experi- 

 ment, it has been done with a view of bringing the re- 

 sults to their simplest conditions. In such inquiries 

 Canton's phosphorus and bodies chemically changeable 

 are wholly inadmissible. 



The specimens of fluor-spar employed by me were de- 

 rived from many different sources, American and Euro- 

 pean. The color of the light they emitted was in some 

 cases blue, in some green, in some yellow. Among them 

 was an American variety of chlorophane of a pale flesh- 

 colored aspect, translucent on the edges, and excelling 

 all the others in the splendor of its light. It equalled 

 the best Canton's phosphorus in power, yielding a superb 

 emerald green light when it received the rays of the sun 

 or of an electric spark, or had its temperature raised. 

 The warmth of the hand in a dark place made it shine. 

 Considering the facility with which we can regulate the 

 intensity of an electric spark, measuring out the quanti- 

 ties of light used in a given experiment, it is clear that 

 there are great advantages in resorting to it in prefer- 

 ence to the variable rays of the sun. Our choice of a 



