582 Dr. A. B. Wood on Long-range 



on such observations. The following example serves to 

 illustrate the method of determining the ratio : — 



(a) Long-range particles (through mica screen of air-equivalent 8 - 6 cm.). 



,, , o • .-n .■ f 8'3 per minute (mean of 15 ob- 



jN umber of scintillations on j r ,. S e -, _. , 



servations each or 1 minute 



Nov. 24, 1919, at 8 p.m. 



duration). 



(b) Ordinary a-particles (mica screen removed, source and zinc-sulphide 

 screen undisturbed). 



Number of scintillations on f 10'2 per minute (mean of 15 ob- 

 Nov. 30, 1919, at 8 p.m. <J servations each of 1 minute 

 (144 hrs. after (a)). ( duration). 



Let N be the total number of ordinary a-particles emitted per minute 

 from the source to the zinc-sulphide screen on Nov. 24 at 8 p.m., 

 and assuming the decav constant \ for thorium active deposit to be 

 0-064 hr.- 1 , we have 



±KJ * — e - -064X144 



N 

 whence N= 102,000 per minute at 8 p.m., Nov. 24, 1919. 

 (c) Ratio. 



Number of long-range particles _ 8*3 1 



Number of ordinary a-particles 102,000 12,300 



The most probable value of the ratio, obtained as a result of all deter- 

 minations similar to this, is 1 in 11,000, a value onlv slight! v lower than 

 that given in. 1916, viz. 1 in 10,000. 



The ratio 1 in 11,000 was obtained with a mica absorbing 

 screen. It has been mentioned, however, in paragraph § 3, 

 that the value observed when an aluminium screen was used 

 was about 7 or 8 per cent, lower than this, viz. 1 in 12,000. 

 If the difference between the two values is real, the result 

 implies that the proportion of u oxygen atoms " from the 

 mica to the total number of ordinary a-particles is in the 

 ratio of 1 to 100,000 approx., a value of the same order as that 

 obtained by Sir Ernest Rutherford when RaC a-particles 

 were fired into the mica. 



Further experiments should be made, however, with a 

 more active source before this point can be definitely 

 established. 



§ 5. Particles of Range greater than 11*3 cm. 

 Hydrogen atoms. 



When RaC a-particles (7*0 cm. range) were absorbed in 

 mica, Sir E. Rutherford showed that, in addition to the 

 oxygen atoms of range 9 cm., a number of hydrogen atoms 



