472 



SCIENCE 



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



is clear from the results that the nuclei of the 

 atoms under consideration can not be re- 

 garded as point charges for distances of the 

 order of the diameter of the electron. Taking 

 into account the close similarity of the effects 

 produced in hydrogen and oxygen, and the 

 greater repulsive forces between the nuclei 

 in the later case, it seems probable that the 

 abnormal forces in, the case of oxygen mani- 

 fest themselves at about twice the distance 

 observed in the case of hydrogen, i. e., for 

 distances less than 7 X 10"^^ cm. Such a con- 

 clusion is to be anticipated on general 

 grounds, for presumably the oxygen nucleus 

 is more complex and has larger dimensions 

 than that of helium. 



In his preliminary experiments Marsden 

 observed that the active source always gives 

 rise to a number of scintillations on a zinc 

 sulphide screen far beyond the range of the 

 a-particle. I have always found these natural 

 scintillations present at the sources of radia- 

 tion employed. The swift atoms producing 

 these scintillations are deflected in a magnetic 

 field, and have about the same range and 

 energy as the swift H atoms produced by the 

 passage of a-particles through hydrogen. The 

 number of these natural scintillations is 

 usually small, and it is very difficult to decide 

 definitely whether such atoms arise from the 

 disintegration of the active matter or are due 

 to the action of the a-particles on hydrogen 

 occluded in the soiu'ce. 



These natural scintillations were studied by 

 placing 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 

 thickness to stop the a-rays 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 corre- 

 sponding 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 the 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 shovm 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-vapor, for the effect ob- 

 served was equivalent to the number of H 

 atoms produced by the mixture of hydrogen 

 at 6 cm. pressure with oxygen. The measure- 

 ments 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 nitro- 

 gen 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 elec- 

 tric field. The experiments are, however, ex- 

 ceedingly difficult on account of the very 

 small number of the scintillations to be ex- 

 pected under the experimental conditions. It 

 should be mentioned that the evidence so far 

 obtained is not sufficient to distinguish defi- 

 nitely 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 col- 

 lision 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 helimn 

 nuclei of mass 4, and either two hydrogen 

 nuclei or one nucleus of mass 2. 



The effect produced in nitrogen would be 



