Rays emitted by Substances exposed to y Rays. 



635 



entering the chamber. In this manner the value 330 was 

 obtained for (Al-tissue paper), corresponding to the value 800 

 for (Pb-Al). The radiation from the tissue-paper must be 

 small in comparison with that from the aluminium plate, and 

 therefore the radiation from aluminium is approximately equal 

 to 330, when the difference in radiation between lead and 

 aluminium is put equal to 800. 



But some of the radiation from 

 objects penetrates the tissue-paper 

 and therefore the true value for 



the air and surrounding 

 and enters the chamber, 



the 



, power of 



radiating 

 aluminium is larger than the above value. Since the secondary 

 cathode rays possess considerable penetrating power (their 

 velocity being about half that of light), some of the rays that 

 penetrate the tissue-paper have passed over a considerable 

 distance in air, and the amount of radiation received from 

 neighbouring objects is therefore probably of such a magnitude 

 that the ionization produced cannot be neglected. The value 

 obtained for the radiating power of aluminium is therefore 

 only approximate, it may be said to be an inferior limit of 

 the radiating power. 



Eve has determined the secondary radiation from a 

 number of substances when exposed to y rays. The values 

 that he obtained are given in Table IV. The y rays were 

 passed through a lead screen 6*3 mm. thick. Since the 

 ionizing power of the secondary y rays is small in com- 

 parison with the secondary cathode radiation, the values 

 express the amounts of secondary cathode radiation from the 

 given substances. 



Table IV. 



Radiator. 



Secondary Radiation. 



Lead 



100 

 61 

 59 

 30 

 35 

 20 



Copper 



Brass 



Aluminium 



Glass 



Paraffin 



The relative values that he obtained for the metals copper, 

 lead, and aluminium, agree with those of the writer as well 



