208 



NA TURE 



[August i i, 1923 



The Origins of the Conception of Isotopes.' 

 By Prof. Frederick Soddy, F.R.S. 



ONE of the most important consequences of the 

 study of the chemistry of the products of radio- 

 active change has been the discovery of isotopes and 

 the interpretation, in consequence, of the Periodic Law 

 in terms of modem views of atomic structure. It is 

 one of the few fields in the vast borderland between 

 physics and chemistry, overrun of recent years by an 

 advancing swarm of mathematicians and physicists, 

 armed with all sorts of new-fangled weapons, in which 

 the invaders have found the chemist already in posses- 

 sion. The broad highways they have hewn thereto are 

 already dusty with the feet of pilgrims and are being 

 watered by the tears of candidates for " Honours." 

 But the somewhat intricate bye-ways through which 

 the chemist first found his way into this virgin territory, 

 and the views on the road before it was in sight, may 

 still preserve something of their pristine interest. 



The word isotope signifies " the same place," in 

 allusion to isotopes occupying the same place in the 

 Periodic Table. Before this word of theoretical mean- 

 ing was coined, isotopes were experimentally well 

 known as elements non-separable by chemical methods 

 and completely identical in their whole chemical 

 character. The analysis of the constituents of matter, 

 to which we were born and brought up to regard as 

 the most searching and fundamental, is an analysis by 

 means of its chemical properties. Although, later, a 

 new and even more powerful method — spectroscopic 

 analysis — was developed, it merely dotted the z's and 

 crossed the /'s of chemical analysis, filled in a few 

 vacant places in the Periodic Law, and handed over 

 the newcomers to the chemist to classify along with the 

 rest of the eighty or so " foundation stones " of which 

 he supposed the material universe to be built up. 



With the close of last century another new method — 

 radioactive analysis — was developed, which is applic- 

 able, of course, only to the radio-elements; that is, to the 

 elements uranium and thorium and the 34, as we now 

 know, successive unstable products of their spontane- 

 ous disintegration. Each of these possesses a definite 

 radioactive character; it is produced from one and 

 changes into another element, and, in both changes, 

 rays characteristic of the two substances are expelled, 

 which are as fine a hall-mark of their identity as any 

 of the " tests " of chemical analysis. But radioactive 

 character, unlike spectroscopic character, is completely 

 independent of chemical character. The latter might 

 be called " existence properties," whereas the radio- 

 active character is that attending the explosion of the 

 atom which terminates the existence of the element as 

 such. It provided the necessary independent method 

 of analysis capable, for the first time, of distinguishing 

 between elements identical chemically and occupying 

 the same place in the Periodic Table, i.e. between 

 isotopes. 



The Earlier Chapter of Radio-chemistry. 



Not a hint of this, however, was afforded by the 

 earlier chapter of radio-chemistry. On the contrary, 

 no development could appear more normal.^ Just as 



' Discourse delivered at the Royal Institution on Friday, May 4. 

 NO. 2806, VOL. 112] 



rubidium, thallium, etc., were detected by the spectro 

 scope before anything of their chemistry was known 

 so radium was detected in pitchblende by its radi«> 

 activity in concentration thousands of times less than 

 is necessary to show a single line of its spectrum. But 

 with more concentrated preparations a new spectrum 

 was discovered, and then a new element, which was 

 found to possess a chemical character entirely new and 

 sufficing for its separation in the pure state from all 

 other elements. As in the case of the elements dis- 

 covered by the spectroscope, radium was found to 

 occupy a place, hitherto vacant, in the Periodic Table. 

 But, as it happened, radium is exceptioniil in this. 

 Its chemical character was quite normal, and indeed 

 could have been largely predicted beforehand for tht 

 missing element occupying this place. The develop- 

 ment of the subject showed it to be but one of some 34 

 radio-elements formed from uranium and thorium. 

 But there are not 34 vacant places in the Periodi* 

 Table to accommodate them. 



Meta-elements. 



So far as I am aware, there is no anticipation, prior 

 to the systematic study of the chemistrj^ of the radio- 

 elements, of the idea that there may exist different 

 elements with absolutely identical chemical character. 

 Sir William Crookes, it is true, once thought, thougli 

 the idea has not survived more extended examination, 

 that the properties of the elements, as we know them, 

 might be a mean value, and that the individual atoms 

 composing the element might differ in weight and 

 chemical character continuously on either side of this 

 mean. If so, more refined methods might serve to 

 resolve the element into a collection of what he termed 

 " Meta-elements," possessing the main character ol 

 the original, but differing from one another to a slight 

 extent. Misled by the phosphorescence spectra, which 

 are now known to be characteristic of mixtures rather 

 than chemically homogeneous substances, he thought 

 at one time that he had been successful in so resohing 

 yttrium. But the present idea, that elements may 

 exist absolutely the same in chemical nature and yet 

 absolutely different in other properties, such as radio- 

 activity and atomic weight, is totally distinct from this. 



The Experimental Method that first 

 revealed isotopes. 



I venture to think that no more elegant extension of 

 our methods of gaining new knowledge has ever been 

 obtained than that which, in due course, was to reveal 

 the existence of isotopes. The original observations, 

 upon which the theory of atomic disintegration was 

 first founded, were that thorium is continuously pro- 

 ducing a new radioactive substance, thorium X, 

 separable from it by precipitation with ammonia but 

 not with other precipitants, and, after separation, 

 continuously re-forming again. The thorium X was 

 short-lived and changed again into a gas, the thorium 

 emanation, for which the name thoron has recently been 

 proposed, which was even shorter-lived and changed 

 again to a solid — the " excited actiNaty " now known 



