at the Royal Institution, 1908-1916 777 



means at our disposal. No sufficient explanation has yet been given 

 of this astounding- behaviour ; chemical precedent would lead us to 

 think that the decomposition may be akin to that of substances 

 which undergo eudothermic change, because the rearrangement of 

 the constituents is one involving a degradation of energy, as in the 

 case of acetylene : — 



X C0H2 = 2 ic C + 2 2! H = 2 C, + ;c Ho. 



But in so far as the production of helium is concerned, this is denied 

 by physicists. 



The occurrence of such changes is known to be conditioned by 

 ''impurity." Highly purified nitrogen chloride is non-explosive ; it 

 may be distilled. If radio-active change be unconditioned, it again 

 is a departure from all other forms of chemical change. 



The problems opened up by the discovery of helium are indeed 

 transcendental. Both chemically and physically it is the least 

 responsive of gases : one in comparison with which hydrogen is emi- 

 nently active, though below it in molecular weight and volatility. 

 The temperature difference between boiling-points of the two 

 materials is relatively small and yet an interval is included within 

 which marvellous changes in the properties of other materials are 

 likely to occur. It is deeply to be regretted that helium is so scarce 

 and that the expense and difficulties attending its use as a refri.L'-erat- 

 ing agent must be so great. So much having been accomplished 

 with hydrogen in the Institution laboratories, it may be hoped, how- 

 ever, that means will yet be found of extending the inquiry into a 

 field of the unknown wherein miracles may well be realized. 



Prof. Soddy has so fully discussed the problem of radio-active 

 change in bis recent fascinating and masterly lecture on " The Com- 

 plexity of the Chemical Elements " that little more can be said with 

 advantage here. We are placed in a difficulty. Since the discovery 

 of radium the term " element " has been deprived of its verbal signifi- 

 cance ; so also has the term " atom." If our language is to be an 

 accurate expression of our thought, either the definition of these 

 terms must be altered or we must devise new ones. 



Apparently the decomposition of radio-active materials may give 

 rise to substances which are indistinguishable in chemical behaviour, 

 although they differ in their combining proportions and some physical 

 aspects. Materials so related are termed isotopes by Prof. Soddy : 

 these correspond to the related or homologous members of a family 

 contemplated in my scheme, which even provides for the existence of 

 isomeric terms in different families. They are comparable also 

 with the homologous terms in a series of hydrocarbons, such as the 

 paraffins. I have suggested that tellurium is the inferior homologue 

 or isotope of a missing " element " bearing the weight-relationship to 



