from the Physico- Chemical Standpoint. 333 



Examination of the figures shows that the intermediate 

 product in the double /3-ray change has an average life very 

 much shorter than those of the parent and the disintegration 

 product. Applying the same reasoning to the case of the 

 salts of thallium, it might be expected that when monovalent 

 thallium loses an electrical charge and passes into divalent 

 tl allium, the latter substance readily loses an electrical 

 charge and changes almost immediately into trivalent 

 thallium, the intermediate stage T1C1 2 being too unstable 

 for isolation. 



Looking at the matter in its essentials, it is clear that both 

 the /3-ray change and the alteration of valency by chemical 

 means produce a marked change in chemical character which 

 is similar in both cases ; and a true theory of the atomic 

 structure must account for these phenomena. 



The model atom described above furnishes a satisfactory 

 explanation of the facts. In their paths, the " cometary " 

 electrons periodically come into close proximity to the 

 positive-negative system of the nucleus ; and while they are 

 in this position they will affect the centre of the atomic 

 structure just as if they were travelling in the innermost 

 negative orbit. In other words, at this stage in their career 

 they behave as if they formed part of that portion of the 

 atom in which the general chemical character is supposed to 

 reside. At the same time, since their presence in this 

 position is only periodic and temporary, they will not exert 

 so much influence as is produced by the electrons of 

 the innermost zone, which are always in touch w r ith the 

 positive electrons and which thus exert a permanent effect 

 upon the atomic character. 



This hypothesis, therefore, accounts for the fact that 

 changes in the valency of an atom induced by chemical 

 reactions do not completely and irreversibly alter its cha- 

 racter as do modifications due to the expulsion of an a- or a 

 /3-ray ; for in the last case the change takes place in the very 

 core of the atomic structure, and its results are deep-seated 

 and permanent. 



Thus the model atom furnishes a solution of the questions 

 arising from the chemical resemblances traceable between 

 the uranous salts and the salts of thorium. The atomic 

 number of thorium is 90, whilst that of uranium is 92 ; so 

 the two elements are not isotopic. Uranium occurs in the 

 hexavalent form and also in another modification which is 

 quadrivalent like thorium. Quadrivalent uranium resembles 

 thorium with a closeness approaching isotopy; but the 

 similarity does not reach the point of identity, since the two 



