470 



SCIENCE 



[N. S. Vol. L. No. 1299 



iisual way. The strong luminosity due to the 

 l8-rays from radium C was largely reduced 

 by placing the apparatus in a powerful mag- 

 netic field which bent them away from the 

 screen. 



If we suppose, for the distances involved 

 in a collision, that the a-particle and hydrogen 

 nucleus may be regarded as point charges, it is 

 easy to see that oblique impacts should occur 

 much oftener than head-on collisions, and 

 consequently that the stream of H atoms set 

 in motion by collisions should contain atoms 

 the velocities of which vary from zero to the 

 maximum produced in a direct collision. The 

 slow-velocity atoms should greatly preponder- 

 ate, and the number of scintillations observed 

 should fall off rapidly when absorbing screens 

 are placed in the path of the rays close to the 

 zinc sulphide screen. 



A surprising effect was, however, observed. 

 Using a-rays of range 7 cm., the number of H 

 atoms remained unchanged when the absorp- 

 tion in their path was increased from 9 cm. 

 to 19 cm. of air equivalent. After 19 cm. the 

 number fell off steadily, and no scintillations 

 could be observed beyond 28 cm. air absorp- 

 tion. In fact, the stream of H atoms re- 

 sembled closely a homogeneous beam of a-rays 

 of range 28 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 pro- 

 jected forward mainly in the direction of the 

 a-particles and over a narrow range of veloc- 

 ity, 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 longer nearly homogeneous. 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, there- 

 fore, 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 pecidiarities, the num- 

 ber of H atoms is greatly in excess of the 

 niunber to be expected on the simple theory. 

 For example, for the swiftest a-rays the mun- 

 ber which is able to travel a distance equiv- 

 alent 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 dimin- 

 ishes rapidly with velocity, and is very small 

 for a-particles of range 2-5 cm. 



It must be borne in mind that the pro- 

 duction of a high-speed H atom by an 

 a-particle is an exceedingly rare occiu'rence. 

 Under the conditions of the experiment the 

 number of H atoms is seldom more than 

 1/30,000 of the number of a-particles Prob- 

 ably each a-particle passes through the struc- 

 ture of 10,000 hydrogen molecules in travers- 

 ing one centimeter of hydrogen at atmos- 

 pheric pressure, and only one a-particle in 

 100,000 of these produces a high-speed H 

 atom; so that in 10^ collisions with the mole- 

 cules of hydrogen the a-particle, on the aver- 

 age, approaches only once close enough to the 

 center of the nucleus to give rise to a swift 

 hydrogen atom. 



We should anticipate that for such collis- 

 ions the a-particle is unable to distinguish 

 between the hydrogen atom and the hydrogen 

 molecule, and that H atoms should be liber- 

 ated from matter containing free or com- 

 bined hydrogen. This is fully borne out by 

 experiment. 



From the number of H atoms observed it 

 can be easily calculated that the a-particle 

 must be fired within a perpendicular distance 

 of 2-4X10-^' cm. of the center of the H 

 nucleus in order to set it in swift motion.. 

 This is a distance less than the diameter of 

 the electron, viz. 3-6X10"^^ cm. The gen- 

 eral results obtained with a-rays of range 7 

 cm. are similar to those to be expected if the 

 a-particle behaves like a charged disc, of 

 radius of about the diameter of an electron, 



