434 Dr. A. S. Eve : A Comparison of the Ionization 



with the magnetic field, show the necessity for this pre- 

 caution. 





No magnetic field. 



Magnetic field. 



Pb 



Ill 



7-8 

 8-0 

 7-6 

 71 

 70 



10-9 



7-6 

 7-6 

 7 "5 

 6 3 

 5-3 



Cu 



Zn 



Fe 



Al 



Cardboard 





The Rontgen rays came from a rather hard bulb (A in air 

 equal to '0004 cm. -1 ), and it was necessary to cut down the 

 intensity of the rays by a plate of lead 1*8 mm. thick placed 

 near the bulb, which was six metres from the electroscope. 

 The time of exposure was governed by a control electroscope. 



Experiments were also made with softer rays, and the 

 results may be contrasted, iron in each case taken as 

 standard. 





Thickness. 



Soft 

 X Kays. 



Hard 

 X Kays. 



Hard X thick- 

 ness X density. 



Hard x thick- 

 ness X atomic 

 weight. 



Pb 



2'1 mm. 

 •54 „ 

 44 „ 

 •54 „ 

 •60 „ 



small 

 6S 

 121 

 100 

 238 



1'6 



82 

 111 

 100 

 126 

 125 



195 



430 

 345 

 393 



400 



780 

 2810 

 3190 

 3080 

 2080 



Cu 



Zn 



Fe 



Al 



Cardboard . . . 



It was necessary to make the lead vessel fairly thick 

 (2*1 mm.) in order to secure rigidity. It was three or four 

 times as thick as the other vessels. The high values for zinc 



e in part due to its relative thinness. 



ai 



Disc 



ussion. 



The problem appears to be too complex for redaction to a 

 simple formula. It is, however, possible to give a satis- 

 factory general explanation of the results, and to show that 

 the ionization effects obtained are not out of harmony with 



the view that X and 7 rays are similar in type. 



The explanation is derived from Bragg's theory o 



f the 



