Dr. S. P. Thompson- on Hyperphosphorescence. 103 



fi M directly but makes interpolations, using the temperature- 

 coefficients which have been determined a good distance 

 from 0°, and so runs into the danger of introducing errors 

 into the calculated degrees of dissociation greater than the 

 differences he has to determine. His results (at 0°) de- 

 viate from Prof. Ostwald's (at 25°) in some cases irregularly 

 by several per cent., and his investigations are limited to 

 three acids and two salts. For this reason the investigation 

 of the matter remains as desirable as it was before. 



Christ Church, Oxford, 

 May 1896. 



V. On Hyperphosphorescence. 

 By Silvanus P. Thompson, JD.Sc, F.R.S* 



THE recent researches of H. Becquerel f on the emission 

 by compounds of uranium and by metallic uranium of 

 invisible radiations which very closely resemble those dis- 

 covered by Wiedemann^ and by Rontgen§, and which yet 

 unquestionably consist of transverse vibrations, are of so great 

 importance that any experiments upon the same line, however 

 incomplete, are of interest to physicists. 



In January last the writer and his assistant Mr. Miles 

 Walker were repeating Rontgen's now familiar experiments 

 on the production of photographic shadows by the emanations 

 from Crookes's tubes, and were casting about for means to 

 shorten the long exposures then necessary, when the idea 

 occurred to them which has independently suggested itself to 

 many other experimenters, namely that of employing fluo- 

 rescent substances in contact with the photographic film to 

 hasten the photographic action by the emission of rays of a 

 visible sort when stimulated by the #-rays. Accordingly, 

 having prepared sheets of paper or of aluminium covered with 

 fluorescent material, they tried the effect of inserting them in 

 some cases below the glass plate, in other cases above, the 

 glass plate with the fluorescent surface next the film, and in 

 yet other cases above the plate but with the fluorescent suiv 

 face outside. The materials so tried were sulphide of calcium, 

 finely powdered fluor-spar, sulphide of zinc (natural blende), 

 sulphate of zinc (artificial), fluoride of uranium and ammo- 

 nium, and sundry platino-cyanides. 



* Communicated by the Author. 

 . t Comptes Jtendus, cxxii. pp. 559, 790, &c. 



X Zeitschrift fur Elektrochemie, ii. p. 159 (Aug. 1895). 



§ Sitzungsberichte der Wurzburger Physih-medic. Gesellschaft, 1895, - 



