produced by (3 Particles from Radioactive Substances. 607 



was calculated graphically, and the values thus found are 

 10*4 cm. and 2*94 cm. respectively. 



In all cases the electro-scopes were standardized in absolute 

 measure, and in addition after each experiment the sensi- 

 tiveness of the electroscope in use was tested by means of a 

 constant preparation of RaD + E which served as a standard. 

 As will be seen from the table, the results obtained with the 

 different electroscopes are in a reasonable agreement. For 

 each active substance three to live experiments were cariied 

 out on each electroscope. 



Radioactive Substances used. 



A few details regarding the radioactive substances used in 

 these experiments may be given. 



Thorium D. — The active material was thorium active deposit 

 on thin aluminium-foil. Account was taken of the fact that 

 two a particles are present in the active deposit. The extra- 

 polation for the initial value of the /3 radiation was made by 

 using the exponential law, taking into consideration, of 

 course, the soft rays of Th A. The absorption of Th D follows 

 an exponential law from the smallest thicknesses of aluminium 

 that could be used up to the thickness which cut down the 

 activity to less than 10 per cent, of the initial value. This 

 was tested by using pure Th D obtained b}' recoil. 



Actinium C. — The active material was actinium active 

 deposit in thin aluminium-foil. The extrapolation for the 

 initial value of the (3 radiation was made by using the expo- 

 nential law. The absorption of the ft rays in this case was 

 also found to obey the exponential law from the smallest 

 thicknesses of aluminium up to those cutting down the radia- 

 tion to less than 10 per cent. Pure actinium C obtained by 

 recoil was used to test this. It may perhaps be well to note 

 that ActB (a rays) and Act C (/5 rays) require considerably 

 different decay factors in the reduction to zero time. In two 

 cases radioactinium preparations were used, and the rise of 

 the /9-ray activity was observed for several weeks. Correcting 

 for the soft (3 radiation of radioactinium and Act X the value 

 of the hard or Act C radiation was obtained at the time of 

 equilibrium. The number of atoms breaking up was calcu- 

 lated from the determination made of the a-ray activity 

 directly after the preparation of the radioactinium plate. 

 Experiments showed that there w r as no Act X present at the 

 time. Both methods gave the same results *. 



* This proves that radioactinium emits the same number of a particles 

 as ActB, a point which has recently been questioned. . 



