1919] on Atomic Projectiles and their Collisions 571 



absorption in their path was increased from 9 cm. to 19 cm. of air 

 equivalent. After 19 cm. the number fell off steadily, and no scin- 

 tillations could be observed bevond 28 cm. air absorption. In fact, 

 the stream of H atoms resembled closely a homogeneous beam of 

 a-rays of range 2S cm., for it is well known that, owing to scattering, 

 the number of a-particles from a homogeneous source begin to fall off 

 some distance from the end of their range. The results showed that 

 the H atoms are projected forward mainly in the direction of the 

 a-particles and over a narrow range of velocity, and that few, if any, 

 lower velocity atoms are present in the stream. 



If we reduce the velocity of the a-particle by placing a metal 

 screen over the source, it is found that the distribution of H atoms 

 with velocity changes, and that the rays are no longei- nearly homo- 

 geneous. When the range of the a-rays is reduced to 3-5 cm., the 

 absorption of the H atoms is in close accord with the value to be 

 expected from the theory of point charges. It is clear, therefore, 

 that the distribution of velocity among the H atoms varies with the 

 speed of the incident a-particles, and this indicates that a marked 

 change takes place in the distribution and magnitude of the forces 

 involved in the collision when the nuclei approach closer than a 

 certain distance. 



In addition to these peculiarities, the number of H atoms is 

 greatly in excess of the number to be expected on the simple theory. 

 For example, for the swiftest a-rays the number which is able to 

 travel a distance equivalent to 10 cm. of air is more than thirty 

 times greater than the calculated value. The variation in number 

 of H atoms with velocity of the incident a-particle is also entirely 

 different from that to be expected on the theory of point charges. 

 The number diminishes rapidly with velocity, and is very small for 

 a-particles of range 2 ' 5 cm. 



It must be borne in mind that the production of a high-speed 

 H atom by an a-particle is an exceedingly rare occurrence. Under 

 the conditions of the experiment the number of H atoms is seldom 

 more than 1/30,000 of the number of a-particles. Probably each 

 a-particle passes through the structure of 10,000 hydrogen molecules 

 in traversing one centimetre of hydrogen at atmospheric pressure, 

 and only one a-particle in 100,000 of these produces a high-speed 

 H atom ; so that in 10-' collisions with the molecules of hydrogen 

 the a-particle, on the average, approaches only once close enough to 

 the centre of the nucleus to give rise to a swift hydrogen atom. 



We should anticipate that for such collisions the a-particle is 

 unable to distinguish between the hydrogen atom and the hydrogen 

 molecule, and that H atoms should be liberated from matter con- 

 taining free or combined hydrogen. This is fully borne out by 

 experiment. 



From the number of H atoms observed it can be easily calcu- 

 lated that the a-particle must be fired within a perpendicular distance 



