372 



Prof. C. G. Barkla on Typical 

 Table I. 



Source of 

 homogeneous 

 X-radiation. 



Absorption of 

 X-radiation 



Ioniz. in C0 2 . 



Ioniz. due to C 2 H 5 Br. 



Ionization 

 in C0 2 at 



76 cm. 

 pressure. 



Ionization 



due to 

 C 2 H 5 Br at 



116 cm. 



pressure. 



Ioniz. in air 



Ioniz. in air 



Fe 



88-5 



59-1 



47-7 



394 



22-5 



18-5 



17-4 



13 



47 



25 



1-57 



12 



1-42 



1-385 



1-39 



1-36 



1-376 



1-35 



1-4 



1-42 



1-38 



1-4 



1-42 



29-5 



30 



30-2 



31-1 



30-2 



29-8 



30-9 



106-7 



133 



153 



175 



166 



[Numbers pu 

 , 1257 



81-9 



66-3 



53-6 



31 



25 



18-2 

 67 

 3-45 

 2-20 

 1-72 



rely relative] 

 571 



387 



315 



268 



148-5 



120 



117-5 



Ni 



Cu 



Zn 



As 



Se 



Br 



Sr 



303 



137 

 83-5 

 60-2 

 44 



Mo 



Ag 



Sn 



Sb 



* The majority of these absorptions were determined by Barkla and Sadler, 

 that for Br radiation by Chapman. 



Column 1 gives the source of the ionizing X-radiation ; 

 column 2 the absorbability of each radiation in aluminium ; 

 column 3 the ratio of ionizations* in carbonic acid and air 

 at the same pressure and temperature ; column 4 the ratio 

 of the ionization due to ethyl bromide in air saturated with 

 ethyl bromide vapour at 0° C. to the ionization in air at the 

 same pressure (11*6 cm. of mercury) and temperature as the 

 ethyl bromide. [Possibly this is not equal to the ratio in 

 pure ethyl bromide and air at the same pressure. The 

 ionizations in vapours when pure and when mixed with gases 

 will shortly be dealt with.] 



An examination of the above results shows that the 

 ionization in carbonic acid gas is proportional to the 

 ionization in air throughout this wide range of penetrating 

 power. Similar results have been obtained with O, SH 2 , 

 S0 2 , N 2 0, and coal-gas, though in some of these the 

 variations from proportionality were greater than in C0 2 . 



* The term ionization is a somewhat ambiguous one. Unless other- 

 wise st ated the relative ionizations in gases mean the relative ionizations 

 in films of the gases, so thin that the change of intensity of the beam 

 of X-rays in transmission is negligible. . 



