lis Professor Frederick Soddy [May 18, 



means, now called isotopes. The natural coroUaiy of this is that the 

 chemical element represents rather a type of element, the members of 

 the type being only chemically alike. Alike they are in most of 

 those properties, ^yhich were studied prior to the last decade of last 

 century and which are due, as we now think, to the outer shells of 

 the atom, so alike that all the criteria, hitherto relied upon by the 

 chemist as being the most infallible and searching, would declare 

 them to be identical. The apparent identity goes even deeper into 

 the region reached by X-ray spectrum analysis which fails to 

 distinguish between tliem. The difference is "^ found only in that 

 innermost region of all, the nucleus of the atom, of which radio- 

 active phenomena first made us aware. 



But, though these phenomena pointed the way, and easily showed 

 to be different what the chemist and spectroscopist would have 

 decided to be identical, it did more. It showed that although the 

 finer and newer criteria, relied upon by the chemist in his analysis of 

 matter, must of necessity fail in these cases, being ultimately 

 electrical in character, yet the difference should be obvious in that 

 most studied and distinctive characteristic of all— the criterion by 

 which Dalton first distinguished the different kinds of atoms — the 

 atomic weight. Those who have devoted themselves to the exact 

 determination of these weights have now confirmed the difference in 

 two separate cases, which, in absence of what perhaps they might 

 regard as " preconceived notions," they were unable to discover for 

 themselves. This is the experimental development to which I wish 

 more especially to direct your attention. It indicates that the 

 chemical analysis of matter is, even within its own province, superfi- 

 cial rather than ultimate, and that there are indefinitely more distinct 

 elements than the ninety-two possible types of element accommodated 

 by the present periodic system. 



The third sense in which the elements are known to be complex 

 is that which, in the form of philosophical speculations, has come 

 down to us from the ancients, which inspired the labours of the 

 alchemists of the Middle Ages, and which in the form of Front's 

 hypothesis has re-appeared in scientific chemistry. It is the sense 

 that denies to Nature the right to be complex, and from the earliest 

 times, faith out-stripping knowledge, has underlain the belief that 

 all the elements must be built up of the same primordial stuff. The 

 facts of radioactive phenomena have shown that all the radio- 

 elements are indeed made up out of lead and helium, and this has 

 definitely removed the question from the region of pure speculation. 

 We know that helium is certainly a material constituent of the 

 elements in the Proutian sense, and it would be harmless, if probably 

 fruitless, to anticipate the day of fuller knowledge by atom building 

 and unl)uilding on paper. Apart altogether from this, however, the 

 existence of isotopes, the generalisation concerning the Periodic Law 

 that has arisen from the study of radioactive change on the one 



