Particles from Thorium Active Deposit. 583 



of range 28 cm. were always observed*. It might be 

 inferred, therefore, that hydrogen particles wonld be produced 

 in a similar manner by thorium C a-particles. 



A careful examination has consequently been made to 

 detect particles of ranges greater than 11*3 cm. For this 

 purpose the source of thorium active deposit was covered 

 with the 8'6 cm. mica screen as before, and with an addi- 

 tional 3"6 cm. mica layer which, with 0*5 cm. of air, makes 

 a total of 12*7 cm. as the air-equivalent between active 

 source and zinc-sulphide screen. In one series of observa- 

 tions only 2 scintillations were observed in 20 "counts," 

 each of 1 minute duration — i. e.. the average number was 0*1 

 per minute, whereas the number of particles whose range 

 exceeded 9 cm. was 8 per minute in the same experiment. 

 On this basis, it appears that the number of particles of 

 range greater than 12*7 cm. probably does not exceed 1 per 

 cent, of the number of range greater than 9 cm., i.e. not 

 more than 1 in 10 6 of the total number of a-particles emitted 

 from thorium C. It is hardly necessary to explain that this 

 value is only very approximate, depending, us it does, on 

 such slender evidence. 



§ 6. Summary of Results. 



The results obtained in 1916 relating to the long-range 

 particles from thorium active deposit have been confirmed. 

 A re-determination of the total number of long-range 

 particles (ranges exceeding 8"6 cm.) to the total number of 

 ordinary a-particles (ranges 5'0 and 8'6 cm.) gives a value 

 1 in 11,000 as compared with 1 in 10,000 obtained in 1916. 



It has been shown that at least 90 per cent, of these 

 particles originate in the active deposit, whilst the remainder 

 are probably produced by intimate collision of a-particles 

 with the oxygen atoms contained in the mica absorbing- 

 screen which was employed to &top the a-particles of 8'6cm. 

 range. Slight evidence has been obtained of the existence 

 of high velocity particles, probably hydrogen atoms, of 

 ranges exceeding 11*3 cm. The proportion of these relative 

 to the total number ot long-range particles is probably not 

 greater than 1 in 100. Thus we have for thorium active 

 deposit : — 



(1) Number of long-range particles \ 



(probably a-rays) relative to the > 1 in 10 4 approx. 

 total number of a-particles. ) 



* Rutherford, Phil. Mag. June 1919. 

 2 Q 2 



