EMANATIONS. 113 



very vexatious and disconcerting and extreme care is neces- 

 sary to prevent the radium giving out rays altogether mis- 

 directed. For days Professor Curie was unable to approach 

 his electrometers or even to enter his laboratory, owing 

 to his acquired radio-activity. These secondary radiations, 

 in the case of zinc, were four times as intense as ordinary 

 uranium. Moreover, this acquired radio-activity cannot be 

 removed by washing. It must be remembered, however, 

 that the radio-activity is only temporary. It vanishes 

 sooner or later upon the removal from the neighbourhood 

 of the potent radium. Fig. 36 is a photograph taken in 



Fig. 36. Phosphorescence induced by induced radio-activity. 



Professor Curie's laboratory, of phosphorescent action 

 caused by these induced secondary radiations. It was ob- 

 tained in the following way : TW T O samples of zinc sulphide, 

 A and B, and a sample of a salt of uranium, C, were placed 

 in cardboard pill-boxes, and these were laid on a metallic 

 plate, under which was the radium chloride. The metallic 

 plate was quite thick enough to be opaque to the Becquerel 

 rays, but under their influence it gave off secondary radi- 

 ations which caused the salts within the pill- boxes to phos- 

 8 



