1921] Isotopes and Atomic Weights 299 



WEEKLY EVENING MEETING, 

 Friday, February 11, 1921. 



The Hon. Sir Charles Parsons, K.O.B. Sc.D. LL.D. F.R.S., 

 Manager and Vice-President, in the Chair. 



F. W. Aston, M.A. D.Sc, 

 Fellow of Trinity College, Cambridge. 



Isotopes and Atomic Weights. 



Possibly the most important generalization in the whole history of 

 chemistry is the Atomic Theory put forward by John Dalton in 1803, 

 and it is a striking tribute to the shrewd intuition of that observer 

 that of his five postulates only one seems to be in the least degree 

 faulty, and over a century of active and unremitting investigation 

 has been necessary to detect the flaw in that one. 



The postulate in question states that " atoms of the same element 

 are similar to one another and equal in weight." Of course, if we 

 take this as a definition of the word " element," it becomes a truism ; 

 but, on the other hand, what Dalton meant by an element, and what 

 we understand by the word to-day, is a substance such as hydrogen, 

 oxygen, chlorine or lead, which has unique chemical properties and 

 cannot be resolved into more elementary constituents by any known 

 chemical process. For many of the well-known elements Dalton's 

 postulate still appears to be strictly true, but for others — probably 

 the majority — it needs some modification. 



The general state of opinion at the end of the last century may 

 be gathered from the following quotations from Sir William Ramsay's 

 Address to the British Association at Toronto in 1897 : " There have 

 been almost innumerable attempts to reduce the differences between 

 atomic weights to regularity by contriving some formula which will 

 express the numbers which represent the weights with all their 

 irregularities. Needless to say, such attempts have in no case been 

 successful. Apparent success is always attained at the expense of 

 accuracy, and the numbers reproduced are not those accepted as the 

 true atomic weights. Such attempts, in my opinion, are futile. Still 

 the human mind does not rest contented in merely chronicling such 

 an irregularity ; it strives to understand why such an irregularity 

 should exist. . . . The idea . . . has been advanced by Prof. 

 Schutzenberger, and later by Mr. Crookes, that what we term the 

 Vol. XXIII. (No. 115) y 



