228 



SCIENCE 



[Vol. LV, Ko. 1418 



to pass through hydrogen the maximum range 

 obtainable for the recoil of hydrogen nuclei 

 is never greater than the equivalent of 29 cm. 

 in air. This makes it clear that the recoil of 

 H particles or protons obtained with nitrogen 

 can not arise from the presence of hydrogen 

 as an impurity in the gas. The H particles 

 must therefore originate in the nuclei of the 

 nitrogen atoms which must therefore suffer 

 disintegration under the intense bombardment 

 of the alpha rays. Results similar to those 

 obtained with nitrogen have been" obtained 

 with other elements that have been examined 

 but it is of interest to note that it is only those 

 elements whose atomic mass is given by 

 4n + 2 or 4n + 3 where n is a whole number 

 that give rise to H particles. Elements of 

 mass 4n like carbon, oxygen and sulphur show 

 no effect. In Table III the results obtained 

 so far are summarized. 



TABLE III 

 Recoil H Particles and Their Ranges 



(c) H particle satellites: backward recoil. 



In experiments with aluminium foils bom- 

 barded by alpha rays it was found that the 

 direction of escape of the H particles was to a 

 large extent independent of the direction of the 

 impinging alpha particles. Nearly as many 

 were expelled in the backward as in the for- 

 ward direction. The maximum range for H 

 particles ejected in the backward direction 

 was, however, found to be less than that of H 

 particles projected forwards. In the case of 

 the former the maximum range was 67 cm. 

 while with the latter it was 90 em. air equiva- 

 lent. 



In order to explain the ejection of H parti- 

 cles in all directions Rutherford and Chadwick 

 have put forward a simple explanation. They 

 suppose that in such an atom as that of nitro- 

 gen the main nucleus has a mass 12 and that it 

 has two H particles moving in an orbit round 

 and close to it. The manner in which the colli- 

 sions are supposed to occvu' is shown in Fig. I. 



(6) Ranges of H particles. 



With aluminium it will be seen the range of 

 the expelled protons is more than twice as 

 great as for those liberated from iiitrogen. 



The number of H particles expelled from 

 the nuclei of the atoms of different elements is 

 found to vary greatly with the speed of the 

 impinging alpha rays. When alpha particles 

 from thorium C which have a range of 8.2 em. 

 in air are used the H particles are relatively 

 numerous. With a particles having a 7 cm. 

 range in air, ?'. e., those emitted by Ra.C, the 

 number of H particles ejected is considerably 

 smaller. With alpha rays of range 5 cm. in 

 air the number is exceedingly small. With 

 aluminium no H particles appear to be re- 

 leased by alpha particles of range less than 

 5 cm. 



A B 



Fig. 1 

 If the collision occurs as in A the H particle 

 is driven in the forward direction of the alpha 

 particle and away from the nucleus; if, as in 

 B, the H particle is driven towards the nucleus; 

 it describes an orbit close to the latter and 

 escapes in a backward direction. The differ- 

 ence in the velocity of the H particles in the 

 forward and backward directions is probably 

 due to the fact that the main nucleus has been 

 set in motion, in the direction of the alpha 

 particle, before the close collision with the H 

 particle occurs. On this view the relative 

 velocity of the H particle and the residual 

 nucleus is the same whether the H particle 

 escapes in the backward or forward direction, 

 but the actual velocity in the backward direc- 

 tion is less. 



(d) Attraction between positive charges. 



