THE MYSTERY OF RADIOACTIVITY 653 



meaning to the terms molecule and atom ; it would be a retro- 

 grade step if chemists were to resume the old practice, especially 

 as two additional terms have been brought into use : that of 

 radicle, applicable both to a single atom and to a group 

 of atoms capable of acting as a whole; and that of ion, to 

 signify the radicle which is active in electrolysis. Moreover, in 

 nearly every case in which Prof. Soddy uses the term atom, 

 the correct term to use is molecule. The atom, it is true, will 

 become more than ever an ideal, if such reservation be made ; 

 but it is an ideal we need. 



The properties of the radioactive elements are most sur- 

 prising in many ways. There have been chemists who have 

 expected doubtless that some day sufficiently powerful means 

 would be discovered enabling us to decompose elements ; 

 no one had dreamt, however, of elements undergoing decom- 

 position spontaneously and thereby themselves affording the 

 long-expected proof of their composite nature ; and no one 

 probably had ever thought of the possibility of such a vast 

 amount of energy being stored up in a substance as is now 

 known to be stored up in Radium. 



How are we to explain the change which it undergoes — is it 

 altogether without analogy, we may ask — is Prof. Soddy justi- 

 fied in asserting that the old laws of chemistry and physics do 

 not suffice ? Of late years, it has been the favourite doctrine of 

 those who dub themselves physical chemists that a great variety 

 of chemical changes are taking place unperceived at very slow 

 rates and that when such changes are caused to take place 

 rapidly by the intervention of a catalyst this but serves to 

 hasten the rate of change. The spontaneous decomposition of the 

 radioactive substances is not surprising from this point of view. 



Nor is it surprising that the change should take place at a 

 constant rate— the behaviour of Radium, in fact, is simply that 

 characteristic of every changing substance : as chemical change 

 always takes place at some constant rate depending upon the 

 conditions. What is remarkable is that we are unable to in- 

 fluence the rate of change— either to retard it or to hasten it by 

 any of the means which are ordinarily effective. Furthermore 

 the amount of energy dissipated is phenomenally large. Where- 

 in lies the explanation of these peculiarities ? 



The first stage in the decomposition of Radium involves the 

 formation of the so-called emanation and of Helium — which are 



