a Particles with Light Atoms. 569 



in producing light on a zinc sulphide screen than an a 

 particle of equal energy. This may be a consequence of the 

 much weaker ionization along the path of the H atom, for 

 since its range is four times that of the a particle and energy 

 •64, the energy spent per unit path is only 1/6 of that due to 

 an a particle. 



In this connexion it is of interest to note, that nitrogen 

 atoms set in motion lyy « particles from radium have a 

 range in air of about 9 cm. Although the energy of the 

 nitrogen atoms after traversing 7 cm. of air is less than that 

 of the H atoms after traversing 12 cm., the nitrogen atom 

 gives a much brighter scintillation than the H atom. 



Probability distribution ofH scintillations. 



In the course of counting H scintillations, it was often 

 noted that a number of the scintillations appeared as instan- 

 taneous doubles, i. e. two points of light of about equal 

 brightness appeared in the field of view at the same instant. 

 Some preliminary experiments seemed to show that the 

 number of these doubles was greater than was to be expected 

 from probability considerations. For example, in counting 

 bright scintillations due to the active deposit of thorium, on 

 an average, about 1*5 doubles per minute were counted for 

 an average of 30 scintillations per minute, while for a similar 

 number of H scintillations the number of doubles was about 5. 

 If these " doubles" from hydrogen were instantaneous doubles, 

 it was obviously a matter of great importance, possibly 

 indicating the disruption by collision of one of the nuclei 

 into two parts. 



A large number of experiments were made to test this 

 question, using both hydrogen and paraffin wax as a source 

 of H atoms, but very similar results were obtained under all 

 conditions of experiment. The most favourable theoretical 

 conditions were chosen to increase the number of such 

 doubles if they existed. For example, the H atoms were 

 liberated in a thin film of paraffin covering an opening 

 1 mm. in diameter, placed near the zinc sulphide screen. 

 The distance of the screen from the paraffin film and the 

 nature of the absorber between was adjusted, so that even if 

 two atoms were shot forward nearly in the same direction, 

 the scattering would separate them on an average a con- 

 venient distance in the field of view of the microscope, 

 which included a field of view 2 mm. diameter. No apparent 

 advantage as regards the number of doubles was gained by 

 this arrangement. 



I was fortunate, in January of this year, to obtain for a 

 Phil. Mao. S. 6. Vol. 37. No. 222. June 1919. 2 R 



