y-Rays of Thorium and Actinium. 145 



Section III. — The relative penetrating powers of the 

 <y-rays of the Radio-elements. 



It is convenient to state at the outset that, as a result of 

 the experiments to be described, the relative penetrating 

 power of the y-rays is, in descending order, thorium D, 

 radium C, mesothorium 2, uranium X, and actinium 0. 

 Thorium D thus gives the most penetrating y-rays known, 

 though the differences between the first four are not great. 

 But although it is easy to arrange the various types of y-rays 

 in order of their penetrating power it is a more difficult 

 matter to assign accurate values for \ to each as their values 

 as we have shown (II. p. 754-755) depend greatly upon the 

 conditions, and their ratio for different rays is by no means 

 constant when the results for several dispositions are com- 

 pared. Thus the values of X for the uranium y-rays in four 

 combinations, of two different metals with two different 

 dispositions, were respectively 46, 28, 25, and 18 per cent, 

 higher than for those of radium, and similar considerations 

 hold good equally when the thorium and radium y-rays are 

 compared. 



The radioactive preparations used in the following measure- 

 ments were : — 



1. Radium : 7 mg. of radium bromide, and in some ex- 



periments 0'5 mg. of radium bromide. 



2. Mesothorium : a single tiny grain of concentrated 



mesothorium obtained from Knofler and Co., equi- 

 valent in y-rays to about 0*31 mg. radium bromide 

 measured through 3 mm. of lead. 



All these preparations are practically point sources in 

 sealed glass tubes. 



3. Radiothorium : this body mixed with moist thorium 



hydrate was contained in a sealed cylindrical tube 

 about 6 cms. long and about 0*5 cm. diameter. It 

 therefore differed from the others in not being a point 

 source. It was equivalent in y-rays to about 0'56 mg. 

 radium bromide measured through 0*3 cm. lead. It 

 also was obtained from Knofler and Go. 



Dispositions. — Sketches of the three dispositions employed 

 are shown in fig. 2. They hardly need further description. 

 Disposition 1 corresponds to that used in obtaining Tables II. 

 and III. (I. pp. 644 & 646). It is easiest to use in practice 



Phil. Mag. S. 6. Vol. 21. No. 121. Jan. 1911. L 



