1919] on Atomic Projectiles and tlieir Collisions 573 



helium atom carries a doul)le charge. In a similar way no certain 

 evidence has been obtained of long-range recoil atoms from lithium, 

 boron, or beryllium. It is difficult in experiments with solids or 

 solid compounds to be sure of the al)sence of hydrogen or water- 

 vapour, which results in the production of numerous swift H atoms. 

 These difficulties are not present in the case of nitrogen and oxygen, 

 and a special examination has been made of recoil atoms in these 

 gases. Bright scintillations were observed in both these gases about 

 2 cm. beyond the range of the a-particle. These scintillations are, 

 presumably, due to swift N and atoms carrying a single charge, 

 for the ranges observed are about those to be expected for such 

 atoms. The scintillations due to recoil atoms of N and are much 

 brighter than H scintillations, although the actual energy of the 

 flying atom is greater in the latter case. This difference in bright- 

 ness is probably connected with the much weaker ionisation per unit 

 •of path due to the swifter H atom. 



The corresponding range of the recoil atoms was about the same 

 in oxygen, nitrogen, and carbon dioxide. Theoretically, it is to be 

 anticipated that the N recoil atom should give a somewhat greater 

 range than the atom. The recoil atoms observed in carbon 

 dioxide are apparently due to oxygen, for if the carbon atoms carried 

 a single charge they should be detected beyond the range of atoms. 



The number of recoil atoms in nitrogen and oxygen 'and their 

 absorption indicate that these atoms, like H atoms, are shot forward 

 mainly in the direction of the a-particles. It is clear from the 

 results that the nuclei of the atoms under consideration cannot be 

 regarded 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 latter case, it seems probable that 

 the abnormal forces in the case of oxygen manifest themselves at 

 about twice the distance observed in the case of hydrogen, i.e. for 

 distances less than 7 x 10"^^ cm. Such a conclusion is to be antici- 

 pated 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 in the sources of 

 radiation 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 thft 

 source. 



