574 Professor Sir Ernest Rutherford [June 6, 



These natural scintillations were studied by placin": the source in 

 a closed box exhausted of air about 3 cm. from an opening in the 

 end covered by a sheet of silver of sufficient tliickness to stop the 

 a-ravs completely. The zinc sulphide screen was fixed outside close 

 to the silver plate. On introducing dried oxygen or carbon dioxide 

 into the vessel, the number of scintillations fell off in amount 

 corresponding with the stopping power of the column of gas. An 

 unexpected effect was, however, noticed on introducing dried air 

 from the room. Instead of diminishing, the number of scintillations 

 was increased, and for an absorption equivalent to 19 cm. of air the 

 number was about twice that observed when the air was exhausted. 

 It was clear from these results that the a-particles in their passage 

 through air gave rise to long-range scintillations which appeared of 

 about the same brightness as H scintillations. This effect in air was 

 traced to the presence of nitrogen, for it was shown in dry, chemically 

 prepared nitrogen as well as in air. The number of scintillations 

 was much too large to be accounted for by the presence of traces of 

 hydrogen or water-vapour, for the effect observed was equivalent to 

 the number of H atoms produced by the mixture of hydrogen at 

 6 cm. pressure with oxygen. The measurements were always made 

 well outside the range of the recoil nitrogen and oxygen atoms, 

 which we have seen are stopped by 9 cm. of air. 



These swift atoms which arise from nitrogen have about the 

 same brightness and range as the H atoms produced from hydrogen, 

 and, presumably, are charged hydrogen atoms Definite information 

 on this point should be obtained by measuring the deflection of a 

 pencil of these atoms in a magnetic and electric field. The experi- 

 ments are, however, exceedingly difficult on account of the very 

 small number of the scintillations to be expected under the experi- 

 mental conditions. It should be mentioned that the evidence so far 

 obtained is not sufficient to distinguish definitely whether these are 

 H atoms or atoms of mass 2, 3, or 4, for the range and brightness of 

 the latter would not be very different from those shown by the 

 H atom. 



It is difficult to avoid the conclusion that these long-range atoms 

 arising from the collision of a-particles with nitrogen are not 

 nitrogen atoms, but probably charged atoms of hydrogen or atoms of 

 mass 2. If this be the case, we must conclude that the nitrogen 

 atom is disintegrated under the intense forces developed in a close 

 collision with swift a-particles, and that the atom liberated formed a 

 constituent part of the nitrogen nucleus. It may be significant that 

 from radio-active data we should expect the nitrogen nucleus of 

 atomic mass 14 to consist of three helium nuclei of mass 4, and 

 either two hydrogen nuclei or one nucleus of mass 2. 



The effect produced in nitrogen would be accounted for if the 

 H nuclei were outriders of the main nucleus of mass 12. The close 

 approach of the a-particle leads to the disruption of its bond with 



