Prof. Draper on the Phosphorescence of Bodies. 85 



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

 though direct experiments show that this is not so, it is much 

 better to resort to fluor-spar, which is free from all such diffi- 

 culties. Besides, this body can be obtained perfectly trans- 

 parent, and also nearly opake ; it occurs of many tints of colour, 

 and can easily be cut and polished to any figure, and had in 

 pieces of any required size. Its phosphorescent powers are very 

 high ; indeed it yields, when properly treated, to no other sub- 

 stance, not even to Canton's phosphorus, in that respect; and 

 greatly exceeds the sulphate of potash, a substance which, how- 

 ever, possesses many eligible qualities. 



It will therefore be understood, that, in selecting fluor-spar 

 and its varieties as the subjects for experiment, it has been done 

 mth a view of bringing the residts to their simplest conditions. 

 In such inquu-ies, Canton^s phosphorus, and bodies chemically 

 changeable, are wholly inadmissible. 



The specimens of fluor have been derived from many different 

 sources, American and European. The colour of the light they 

 emit is very variable, some yielding a blue, some a green, others 

 a yellow. Among them there is an American variety of chloro- 

 phane, of a mdky or pale flesh-coloured aspect, translucent on 

 the edges, and excelling all the others in the splendour of the 

 light it emits. It equals the best Canton's phosphorus in power, 

 yielding a superb emerald-green light when it receives the rays 

 of the sun, or of an electric spark, or has its temperature raised. 

 Tlie warmth of the hand in a dark place makes it shine. Con- 

 sidering the facility with which we can regulate the intensity of 

 an electric spark, measuring out the quantities of light used in 

 a given experiment, it is clear that there are great advantages in 

 resorting to it in preference to the variable rays of the sun. Our 

 choice of a substance should be controlled by these circumstances, 

 and fluor-spar completely fulfills all the indications. 



It will be seen, however, that I have not restricted myself to 

 the use of this body, but Mdierever other substances could be 

 compared with it, have resorted to them also. The general 

 principles here set forth as applicable to fluor-spai', may be like- 

 wise extended to them. 



To what cause are we to attribute phosphorescence ? what are 

 the changes taking place in the glowing body ? 



We have already seen that a century ago two difierent 

 answers had been given to these questions. Lemery supposed 

 that bodies act towards light as they do to heat, absorbing it and 

 then giving it out ; Dufay, that all phosphorescences arc cases 

 of combustion. 



