a. Particles with Light Atoms. 555 



Then obviously 



number of H atoms per second on area A== ~ = ^ — ~. 



60 ±ttI~ 



Taking / = 2'85 cm., A ==0*84 sq. mm., n=3'72xl0 7 J 



then p = 9'7xl0- 6 , 



or in round numbers p=10~ 5 . 



This number was obtained for a total absorption in path of 

 H atom corresponding to about 15 cm. of air, but we have 

 already seen that the number under conditions of experi- 

 ment does not vary sensibly between 9 and 19 cm. absorption. 

 We have seen in § 7 that the number of H atoms to be expected 

 on the simple theory is "98 x 10" G for 10 cm. absorption and 

 *31 x 10~ 6 for L9 cm. We thus see that for an absorption of 

 10 cm., the observed number of H atoms is 10 times the 

 theoretical value and for 19 cm. 31 times. 



Using the observed result that 1 in 10 5 of the a particles 

 produces one H atom per centimetre of path of hydrogen, 

 it is easy to calculate the maximum distance of the direction 

 of flight of the a particles from the centre of the hydrogen 

 atom in order to produce a high speed atom. 



If p = this perpendicular distance, 



N = number of atoms of H per c.c. at N.T.P. 



Then 7rp 2 N = 10- 5 . 



Taking N=2x 2-705 X 10 19 , 

 then p = 2'4xl0- 13 cm., 

 or, on an average, each a particle of radium C of range 

 7 cm. produces an H atom when the perpendicular distance 

 of its path from the centre of the H atom is equal or less 

 than 2*4 x 10" 13 cm. It should be remembered that this 

 calculation deals with the a particles of range 7 cm. when 

 the H atoms are projected mainly in the direction of the 

 incident u particles and with a range not less than 19 cm. 

 of air, i. e., with a velocity comparable with the maximum 

 velocity of the H atom. As already shown, the distribution 

 of velocity is very different for a particles of shorter range, 

 although the aetual number in all cases exceeds considerably 

 the value calculated on the simple theory. 



§ 11. Closeness of approach of a particles to H nucleus. 



The experimental results considered show that the number 

 and distribution of H particles are very different from those 

 calculated on the assumption that the a particle and H atom 

 are to be regarded as point nuclei carrying charges +2e and 

 + e respectivelv, and indicate that the forces involved in 



2Q2 



