Radium, Thorium, and Actinium in Electric Fields. 375 



is dissociated by a recoil atom, a free atom of oxygen and one 

 of carbon will be produced. The beat of formation of bi- 

 molecular oxygen from atomic oxygen has been estimated 

 by Lewis * to be about 14 x 10 4 calories per gram-molecule. 

 The work done, therefore, in dissociating single molecules 

 will be much greater than when the dissociation is bi- 

 molecular. It is considered, therefore, that the dissociation 

 of a gas by heat is only an approximate guide to the stability 

 of the gas under the bombardment of recoil atoms. 



A qualitative explanation of the results obtained may be 

 given if the following assumptions are made : — 



1. At a collision between a recoil atom and a molecule the 

 former may become positively charged if neutral before 

 the collision, if the relative velocity of the two is greater 

 than a certain value, which depends on the nature of the 

 recoil atom and of the gas. 



2. If the relative velocity is greater than this amount, the 

 recoil atom has a large chance of being positively charged 

 immediately after a collision in which the gas molecule is 

 not dissociated. If positive before such a collision, it 

 generally retains its charge. 



3. At a collision which dissociates the gas molecule, the 

 recoil atom has a large chance of becoming neutral. 



According to these assumptions, the last collision causing 

 dissociation may leave the recoil atom neutral, and may 

 reduce its velocity to zero or to an amount which is too 

 small to allow the recoil atom to ionize itself and become 

 positively charged. Hence some neutral atoms are likely 

 to be present in all gases except those which are monatomic. 

 If the minimum velocity at which dissociation can be effected 

 is much greater than that at which the recoil atom can ionize 

 itself, then its chance of being positive at the end of its path 

 is large. If, however, the former veloeitv is less than the 

 latter, all the atoms would be neutral if we were to assume 

 that dissociation occurs at every collision, provided the 

 velocity is sufficient and that the atom is never positive after 

 such a collision. These two assumptions are not at all 

 likely to be correct ; but if the former velocity is very much 

 less than the latter, values approaching zero would be 

 expected from the assumptions previously made. In support 

 of assumption (3), it is suggested that on the disruption of a 

 molecule one or more electrons may be freed which arc 

 liable to be captured by a positively charged recoil atom. 

 Also, a head-on dissociating collision will probably be 

 inelastic, the recoil atom penetrating the molecule and 

 * Lewis, J. Chem. Soc. Trans, cxv. p. 182 (1919). 

 2C2 



