The Eighteenth Century 189 



for oxygen, while C. P. D. Beckerhinn (1789) spoke of " reiner 

 Luft." The remainder adopted modern terminology. Their results 

 will be found in the chapters dealing with the appropriate lumi- 

 nescence. 



Thus the eighteenth century was to see the final passing of the 

 Greek elements— earth, air, fire, and water— as simple principles. 

 Earth was obviously of complex composition. The studies outlined 

 above indicated that air could no longer be regarded as a simple 

 gas. Lavoisier's views on combustion placed fire in proper perspec- 

 tive, although heat was regarded as a material substance until Count 

 Rumford's experiments in 1798. Water had been formed by com- 

 bustion of inflammable gas (hydrogen) with air in the experiments 

 of Cavendish (1784) , but his explanation was a strained application 

 of the phlogiston theory. It remained for Lavoisier (1784) to set 

 the matter straight and to show that " hydrogene " and " oxygene " 

 (his own names) formed water by combustion. The last of the 

 Greek elements was proven to be a compound. All this work pre- 

 pared the way for a much more rational approach to the explanation 

 of luminescences. 



Crystalloluminescence 



At the beginning of the eighteenth century every type of lumi- 

 nescence had been recognized except crystalloluminescence, the light 

 which appears when certain solutions crystallize, and radiolumines- 

 cence, the light from bombardment of matter with particles, with 

 X-rays, or with gamma rays. Radioluminescence was a discovery of 

 the next century but crystalloluminescence became known from the 

 simultaneous observations of J. G. Pickel (1751-1838) and of Schon- 

 wald, in 1786. Both these men saw the striking greenish light which 

 appears when solutions of potassium sulphate crystallize rapidly. 

 The phenomenon is fairly widespread, but has not been too exten- 

 sively studied and is not well understood, even at the present time. 

 Details of the early and later experiments must be left for Chapter X. 



Luminescence iyi Literature 



Many passages from non-scientific literature might be quoted to 

 illustrate the early interest in glowworm and firefly light, as well 

 as folklore concerning these insects. From Dante's, " Fire-flies in- 

 numerous spangling o'er the vale " {Inferno, XXVI, 29, 1300) , to 

 Du Bartas' verses on the cucuyo in La Creation du Mond (1578) , 

 and Shakespeare's lines in Pericles (II, 3, lines 43-44, 1609) , 



. . . like a glow-worm in the night, 



The which hath fire in darkness, none in light. 



