106 Dr. S. P. Thompson on Hyperphospliorescence* 



absorbed by bone than are the #-rays emitted from denser 

 materials such as platinum. 



. At an early stage of these investigations the use of a fluo- 

 rescent screen revealed the fact that the relative transparency 

 of flesh and bone differed with different materials used as 

 emitters, and depended also upon the degree of exhaustion. 

 The necessary inference that the x-vays are not all of one kind, 

 but are heterogeneous, was announced by the writer about 

 the same time* that the same conclusion was drawn by MM. 

 Benoitf and Hurmuzescu from other causes. To the rays 

 emitted from apatite, bone w T as indeed found to be more opaque 

 than to those emitted from platinum. But apatite, when 

 subjected to the kathode discharge, continues to give out 

 gases which after a very few seconds spoil the vacuum ; and 

 the tube containing apatite as an anti-kathode could not, con- 

 sequently, be used except attached to the pump. Glass was 

 found to be more transparent to #-rays emitted from platinum 

 than to a?-rays emitted in the same tube from glass. 



The extraordinary property exhibited by the uranium com- 

 pounds of emitting a persistent invisible radiation that will 

 pass through aluminium and produce photographic action 

 w r ould suggest that these rays are identical with Rontgen's, 

 were it not that BecquerePs success in reflecting, refracting, 

 and polarizing them proves that they are more akin to ultra- 

 violet light. The latter does not indeed penetrate aluminium: 

 but it has long been known that ultra-violet rays penetrate 

 films of silver which though thin are thick enough to reflect 

 all visible kinds of light. It would seem to be proved, then, 

 that Becquerel's rays differ from the known ultra-violet in 

 degree rather than in kind, being rays of higher frequency 

 and shorter wave-length. That their properties are inter- 

 mediate between those of ultra-violet and of the Rontgen 

 rays furnishes a strong presumption that the latter also differ 

 only in degree, and are an extreme species of ultra-violet 

 light. It should not be forgotten that so far back as 1857 

 M. Niepce de Saint Victor observed many cases in which an 

 object, an engraving on paper or a figured piece of porcelain 

 or marble, immediately after exposure to sunlight, was found 

 capable of giving a photographic impression to a sheet of 

 paper prepared with chloride of silver, with which it was 

 placed in contact. He even used, after exposure to light, 

 cardboard imbibed with salts of uranium or with tartaric acid, 

 and found such to be capable of emitting rays that were 

 photographically active. There was no attempt made, how- 



" * Comptes JRendus, cxxii. p. 807. f Ibid. cxxii.p. 779. " 



