a Particles ivith Light Atoms. 553 



passing through absorbers of known stopping power. The 

 results are given in the table below. 



Absorption 

 of gold foil 

 in terms of 

 cms. of air. 



cm 

 17 „ 

 2-5 „ 

 3-3 „ 

 40 „ 



Issuing range 



of a rays 

 in cms. of air 



7 cm. 

 53 „ 

 45 „ 

 3-7 „ 



3-0 ., 



Absorption 

 in terms of air 

 between source 



and screen 



including 



the hydrogen. 



8-3 cm. 

 8-3 „ 

 7-5 „ 

 6-6 „ 

 6-6 „ 



Observed 



Calculated 



number of 



number of 



H atoms. 



H atoms. 



100 



100 



77 



103 



51 



119 



25 



139 



5 



128 



In the last column are given the relative numbers of 

 H atoms to be expected on the simple theory given in § 7, 

 when account is taken of the maximum range of the H atoms 

 and the thickness of absorber traversed. Since the number 

 of H atoms for corresponding ranges varies as the inverse 

 fourth power of the velocity of the a particles, i. <?., as the 

 inverse four thirds power of! the effective range of the 

 « particles, the number of H atoms should increase with 

 lowering of the velocity of the incident a particles. The 

 observed numbers, however, instead of increasing with re- 

 duction of velocity of a. particles, fall off slowly at first and 

 then very rapidly for ranges between 3*5 and 3 cm. 



In these experiments, the intensity of the H radiation was 

 reduced by this passage through absorbing material equal to 

 fi-G cm. of air. In order to reduce this absorption, another 

 series of experiments was made in which the silver plate of 

 stopping power 5'8 cm. was replaced by an aluminium 

 plate of stopping power 3*7 cm. The velocity of the 

 a particles was reduced by aluminium foil instead of gold 

 foil, placed close to the source. The aluminium foils used in 

 these experiments were freed as far as possible from hydrogen 

 by heating in a vacuum, and the results obtained with 

 aluminium as absorber were similar to those with gold. The 

 presence of numerous H atoms was observed for a particles 

 of ranoe 2'5 cm., but the number was small and just 

 measurable with certainty for « particles of range 2*0 cm.. 

 The actual number in the latter case was small compared 

 with that observed for a particles of range 3 cm. Experi- 

 ments at low ranges are rendered somewhat difficult by the 

 necessity of taking into account the H scintillations which 



Phil. Mag. S. 6. Vol. 37. No. 222. June 19 L9. 2 Q 



