506 Mr. G-. T. Beilby. Phosphorescence caused by [Jan. 25, 



blows is rather complicated, and it is difficult to deduce from the results 

 anything more than the general conclusion stated above, that the wire 

 is substantially elastic up to stresses much beyond the static elastic limit, 

 and that the mathematical theory gives correct results. I hope, how- 

 ever, by suitable modifications of the experiment, to simplify the con- 

 ditions, and obtain by this method more detailed information as to the 

 properties of materials when subjected to shock. It seems to me quite 

 possible that the stress-strain relations for stresses beyond the elastic 

 limit may be much simplified if the stresses are applied for exceedingly 

 short times, because the complication of hardening, due to over- 

 straining, will be to a large extent removed. 



" Phosphorescence caused by the Beta and Gamma Kays of 

 Kadium." By G. T. BEILBY. Communicated by Professor 

 LARMOR, Sec. RS. Eeceived January 25, Kead February 9, 

 1905. 



1 . Various observers have noticed that barium platino-cyanide, after 

 continued exposure to the rays from radium, becomes brown or red, 

 while the phosphorescence excited by the rays falls off considerably.* 

 The following observations were made with the object of ascertaining 

 the conditions under which this change occurs. 



2. A specimen of Merck's barium platino-cyanide was recrystallized 

 And obtained in prisms from 3 to 5 mm. long. The crystals were 

 bright canary yellow and showed a pale blue fluorescence by obliquely 

 reflected light. 



3. The radium used was 30 milligrammes of pure bromide contained 

 in a cell with a thin mica cover. This radium is the property of 

 Mr. Frederick Soddy, to whom I am much indebted for granting me 

 its exclusive use for more than three months. All the experiments 

 were made without removing the mica cover of the cell, so that the 

 effects produced were due entirely to the /5 and y rays. In comparing 

 the phosphoresence at different stages, black paper was interposed 

 between the cell and the substance so as to cut off" the luminous rays 

 from the radium. 



4. When yellow crystals of platino-cyanide are left on the mica 

 cover of the radium cell for half an hour, the beginnings of the colour 

 change from yellow to red are distinctly visible. In one hour those 

 surfaces most directly in the path of the rays become strongly reddened. 

 In eight hours the phosphorescence has fallen to a minimum of 8/100 of 

 its original amount, at which it remains, however long the exposure 

 may be continued. 



Crystals were exposed to the rays for eight days, but the phosphor- 



