Phosphorescence 327 



he had previously noted no " phosphorism." He often concentrated 

 the sunlight by means of a lens and came to the conclusion that all 

 plant and animal parts were phosphors if they were well dried, 

 even milk ^^ and fat " if hardened by cold." The flesh of animals 

 was least phosphoric but became so when well dried. Beccari was 

 interested to know whether living material was phosphoric and 

 investigated his own hand: " I saw, to my apprehension, some faint 

 traces of light upon my fingers especially at the sides and ends." 

 He thought the effect might be due to dust or dirt, but subsequent 

 observations after his hand had been thoroughly cleaned also re- 

 vealed phosphorescence. One January during a severe frost his 

 newly washed hand shone particularly bright and Beccari concluded 

 that " the human skin proved upon good grounds to be phosphoric." 

 Again the only substances that showed no trace of phosphorism 

 under any conditions were metals. " Indeed I have never desisted 

 from trying every means I could devise to make them phosphoric, 

 but never with success." He came to the conclusion that a metal 

 was " surely a hard and stubborn kind of body, since it rejects light 

 and electricity and even dew." 



Charles Francois de C. Dufay 



The next investigation of real importance was by the French 

 savant, Charles Francois de Cisternay Dufay (1698-1739) who in 

 1724 had tested a number of minerals, precious, and semiprecious 

 stones (see Chapter IX) , finding that they would luminesce on 

 warming, but if cooled and then heated again there was no light. 

 These stones exhibited a thermophosphorescence and their lumi- 

 nescence with rise in temperature depended on a previous exposure 

 to light. His complete paper was published in 1726. 



In a second paper in 1730 Dufay (1732) demonstrated that many 

 calcium-containing materials such as belemnite, gypsum from Mont- 

 martre, marble, ivory, bones, egg shells, and oyster shells treated 

 with nitric acid and then calcined, were converted into phospho- 

 rescent material. Topaz (aluminum silicate) also became a phos- 

 phor when treated in this manner. The color of the emitted light 

 was different with different stones. 



In a third paper in 1735, Dufay (1738) made an exhaustive study 

 of the diamond and other precious stones, giving the history of 

 luminous gems and summing up his ideas on luminescence in gen- 

 eral. He established the fact that two yellow diamonds would lumi- 

 nesce visibly for at least twelve minutes after exposure to daylight 



^^ Beccari may have tested his milk frozen. 



