394 Prof. Thomson, On the presence of 



a closed lead vessel with a gold-leaf electroscope inside ; the vessel 

 was provided with a large aluminium window through which the 

 rays passed before falling on lead, the lead was exposed on several 

 occasions to strong Rontgen rays for four hours consecutively, but 

 after stopping the rays the rate of collapse of the gold leaves was 

 the same as before the exposure, showing that either no emana- 

 tion had been formed or if formed had not lasted over the interval 

 between the stoppage of the rays and the observation of the elec- 

 troscope. As the observation of the electroscope occupied several 

 minutes a very evanescent emanation might escape detection by 

 this method. I therefore examined whether any induced radio- 

 activity was produced. For this purpose a long wire was electri- 

 cally connected with the gold leaves and maintained at a negative 

 potential of 1000 volts during the exposure to the rays, but again 

 after the exposure had ceased the rate of collapse of the gold 

 leaves was the same as before, thus if any emanation is given off 

 by the lead the life of both the emanation and the induced radio- 

 activity due to it must be exceedingly short. 



I also tried whether a salt of uranium could be made to give 

 off an emanation by exposure to the radiation from radium. To 

 test this 15 mg. of pure radium bromide were sealed in a thin 

 glass tube and placed in a solution of the uranium salt, but 

 again after an exposure of several days no emanation could be 

 detected in the solution. 



Though no evidence has been obtained that the property of 

 giving off an emanation is at all general, there is I think a con- 

 siderable amount of evidence that most if not all bodies are 

 continually emitting radiation which like the Rontgen rays can 

 ionize a gas through which it passed. This evidence is derived 

 from the study of the ionization of a gas shut up in a closed 

 vessel. M c Clennan and Cooke have observed that the ionization 

 inside a closed vessel is diminished when the outside of the vessel 

 is surrounded by lead, the maximum amount of reduction being 

 about 30 per cent. A layer of lead about 2 cm. thick was sufficient 

 to produce the maximum reduction, no further reduction taking 

 place even when as in Cooke's experiments nearly a ton of lead 

 was placed outside the vessel. This result shows that part of 

 the ionization is due to radiation coming from outside ; having 

 regard to the wide distribution of radium and its emanation the 

 most obvious view is that the radiation arises from radium and 

 its emanation diffused through the air and through the soil. A 

 closer study of the effect of screens on the ionization inside a 

 closed vessel will I think show that this explanation is insufficient. 

 Let us consider what would be the intensity of the radiation 

 coming from a large mass of substance containing a radio-active 

 impurity. In consequence of the absorption of the radiation by 



