the Absorption of X-Rays. 



167 



the bulb. The same bulb, a Siemens' tungsten bulb of 

 Grundelach type, with fin radiator for cooling the anti- 

 cathode, has been employed in all experiments. But as 

 the composition of radiation, in the case of an intermittent 

 current, depends not only on tiie potential difference at the 

 bulb but also on the gas-pressure in it, the greatest pre- 

 caution has been taken to keep the gas-pressure as constant 

 as possible. For this purpose the current in the bulb has 

 been steadily controlled by means of a milliamperemeter, 

 and by a careful and continual regeneration of the bulb 

 it has been possible to keep the current very nearly constant. 

 That the composition of the radiation has not undergone 

 any alterations such as to have appreciably influenced the 

 results is evidenced by the fact that different experiments 

 repeated on different occasions with one and the same element 

 have given very nearly the same results. 



Both absorption vessels (E and F in fig. 1) having a 



-220 volt f=Eb 



Electrometer — j» y /ft — ( 



+220 volt ;> I 



diameter of 3 cm. are placed at a distance of 12 cm. from 

 their respective ionization chambers, whose front is made of 

 lead plates, in which there are round openings of a radius 

 of 2*5 cm. covered with mica. The position of the bulb 

 has been adjusted so that with the same thickness of the 

 layer and the same liquid in the two absorption vessels 

 the intensity of both the ray-pencils penetrating into the 

 ionization chambers should be as nearly alike as possible. 

 The distance from the focus to the ionization chambers is 

 42 cm. Between the absorption vessels and the bulb there 



N2 



