824 Sir E. Rutherford and Mr. J. Chaclwick on the 



a particle are 1*404, '007, '036 for the H atom, nucleus, and 

 a particle respectively after a collision in which the H atom 

 is shot forward. The corresponding numbers when the 

 H atom is shot backward are 1*13, '24, "036 respectively. 



The total gain of energy of the parts after the collision is 

 '45 the energy of the incident « particle. 



These deductions, which apply only to the case of an 

 a particle of range 7 cm., are of the order of magnitude to 

 be expected. Possibly the excess energy may be ascribed to 

 a rearrangement of the nucleus which accompanies the dis- 

 integration. Unfortunately, the number of H atoms, if any, 

 emitted backwards from nitrogen is too small for accurate 

 determination of their maximum velocity. From similar 

 calculations to those given above, it can be shown that the 

 system neither gains nor loses energy if the H atoms have 

 a range of 19*2 cm. backwards when the forward range is 

 40 cm. If the backward range is relatively greater, there 

 would be a gain of energy by the disintegration. 



General considerations. 



The chance that an a. particle is able to liberate a switt 

 H atom from a nucleus is exceedingly small. In the case of 

 aluminium, for example, the number of scintillations ob- 

 served on a screen of 8*3 sq. mm. area for an absorption of 

 30 cm. is about one per minute per milligram for a bom- 

 barded plate of aluminium of 3'5 cm. stopping-power, 3 5 cm. 

 distant from the screen, and for a. rays of initial range 7 cm. 

 Taking the efficiencv of the screen as *75, it follows that onlv 

 two <x particles in one million are able to liberate a swift 

 H atom. Knowing that the a particles from one gram of 

 radium produce only 168 cubic mass of helium per year, it 

 is easily seen that the volume of hydrogen per year liberated 

 from nitrogen or aluminium under practical conditions 

 would be exceedingly small. 



The observations on the disintegration of the light elements 

 have been interpreted by supposing that the H atoms are 

 satellites of the central nucleus. This implicity assumes that 

 positively charged bodies attract one another at the very 

 small distances involved. Such attractive forces must exist 

 in order to hold the ordinary composite nucleus in equi- 

 librium, and it seems likely that these attractive forces will 

 extend some distance from the nucleus. If this view be 

 correct, the forces on the « particle are initially repulsive, 

 but change sign very near the nucleus. As the law of force 

 very near such nuclei is unknown, it is difficult to form any 



