138 Bainboiv Phosphorescence. 



the Bologna phosphorus emitted one shade of colour without 

 reference to the part of the solar spectrum to which it was 

 exposed. 



In 1 764, Canton introduced the so-called phosphorus bearing 

 his Dame, and as his method was easy to follow, it was very 

 generally adopted. He calcined oyster-shells in a crucible, 

 powdered the resulting substance, mixed it with one-fourth its 

 weight of sulphur, and kept it a red heat for one hour. The 

 result of this process is a sulphide of calcium, which, after 

 exposure to light, shines in the dark with a green or yellow 

 lustre, according to the details of its preparation. " Canton," 

 observes M. Becqnerel, " showed that the light of a candle, of 

 the moon, or of electric sparks, rendered this substance active. 

 He also showed that heating it in the dark did not render it 

 luminous unless it had been previously exposed to the light, 

 and that if heated immediately after such exposure, its action 

 was more energetic than when it was heated a few days after- 

 wards. Thus, heat occasions a rapid emission of light that 

 would have been very slowly given forth at ordinary tempera- 

 tures. Canton also showed that if at the end of several months 

 this substance, previously insolated, was heated with boiling 

 water, it gave no result ; but when the heat was carried to a 

 temperature of about 500° (C), it became luminous, and then' 

 fell back to its inactive state until revived by a fresh insolation.'" 



In 1780 Wilson published his experiments on the prismatic 

 colours exhibited by phosphorus. He noticed that different 

 portions of calcined oyster-shells exhibited different phospho- 

 rescent tints, red, yellow, green, or blue, and he separated them 

 accordingly. Wilson also confirmed the statements of Dufay 

 and Zanotti, that each portion of his calcined shells emitted 

 their peculiar light, whatever might be the colour of the 

 exciting rays ; thus his red luminous fragments, he said, 

 emitted a red light in the dark, whether placed in the violet, 

 the blue, or the red of the spectrum, and the luminous portions 

 exhibited a green phosphorescence after the action of violet, 

 blue, or red rays. M. Becquerel remarks that these experi- 

 ments were partly vitiated by the employment of an imperfect 

 spectrum. His own researches show that the blue and violet 

 rays act more energetically than the red, and that with Can- 

 ton's phosphorus the red not only give no illumination, but 

 exert a destructive power. A similar observation had been 

 previously made by Goethe and Seebeck, and M. Becquerel 

 cites the following passage from the work of the former on 

 colours : — " The Bologna phosphorus becomes luminous under 

 the influence of blue and violet glass, and never under yellow 

 or orange glass ; and it may be remarked that this phosphorus 

 rendered luminous by blue or violet colours becomes extin- 



