202 



DISCOVERY 



time more important, namely, the experimental proof 

 that his clement ionium W£is the parent substance of 

 the element radium. 



Three years later the work was taken up by two 

 Germans working in Berlin, Professor Marckwald and 

 Dr. Kectman. The interest of these men was primarily 

 in analytical chemical work. Like Boltwood they 

 found ionium and thorium very similar in their 

 chemical properties, but they concluded, after two 

 years' fruitless attempt to concentrate one even in 

 the least degree in a mixture of both, that ionium 

 and thorium not merely resemble one another in 

 a remarkable way, but do not in fact differ at all 

 in any of their chemical properties. This was a 

 remarkable conclusion, and the point now arises : Is 

 this really so, or simply a confession of failure ? It 

 has not seldom happened that one man, in attempting 

 to separate two of the difficultly separable rare-earths, 

 has failed, while another attempting the same problem 

 has succeeded . Might not this apply here . The answer 

 is " No." Every conceivable chemical method was 

 tried by these two experts. To detect the slightest 

 change they had an instrument which was many 

 thousands of times more sensitive than any used by 

 those who worked on the rare-earths, yet change there 

 was none. This was a most important result, and 

 Marckwald must be regarded as the pioneer in this 

 work on isotopes. 



Shortly afterwards Dr. Hahn, another worker in 

 Berlin, showed that an element mesothorium discovered 

 by him had identical chemical properties with radium, 

 and also that radiothorium, another of his discoveries, 

 was identical chemically with both thorium and 

 ionium. Now thorium has an atomic weight of 232, 

 radiothorium of 228, ionium of 230 ; their radio-active 

 properties are very different, yet their chemical proper- 

 ties are identical ! 



The next move forward was in 1912, when a research 

 student in Professor Rutherford's laboratory in Man- 

 chester put forward the theory that these chemically 

 similar elements (non-separable elements as they were 

 then called) were identical also in physical properties. 

 This was a somewhat revolutionary view at the time, 

 for though it may be very well to talk about bodies 

 being similar chemically, to say that two different ele- 

 ments have, for example, the very same spectrum was 

 a speculation that required proof from more than one 

 source before it could be seriously entertained. It is 

 well known that the spectrum of an element is a very 

 characteristic property. Several elements have been 

 discovered by means of their spectra. The presence of 

 one per cent, of an impurity in a preparation is easily 

 detected by means of its spectrum. If you can show, 

 therefore, that two bodies have the same spectrum, you 

 are demonstrating that two bodies which appear to be 



identical from their chemical properties are in reality 

 identical. Yet it was found at Manchester that thorium 

 and ionium in a preparation containing as much as 

 twenty-five per cent, of the latter had identical sf)ectra. 

 This result has since been confirmed by other workers 

 and is now established. 



This work on isotopes was not only interesting in 

 itself. It led to a great simplification in our knowledge 

 of the chemistry of the radio-active elements. One of 

 the most important discoveries in Inorganic Chemistry 

 in the nineteenth century was the classification of the 

 elements known as the Periodic System. This classi- 

 fication has been described in one of the early articles 

 in this journal.' The radio-active elements number 

 about forty, but the places in the periodic system into 

 which they must fit, unless they are to be classed as 

 exceptional, number but twelve. Now the periodic 

 system is too true a representation of facts to be lightly 

 set aside ; also experimental work has shown that these 

 elements are not exceptional. A difficulty thus arises. 

 How is it to be met ? At first this problem was not 

 faced. Workers in radio-activity were so engrossed 

 in investigating the radio-active properties of these 

 curious and interesting substances that an investigation 

 of their chemical properties seemed irrelevant. The sub- 

 stances existed in such microscopic amounts, the most 

 of them, that the early workers were not sure whether 

 these bodies could be said to have chemical properties 

 in the ordinary sense at all. Nor did they care. Any 

 slap-dash method of separating them from minerals or 

 from preparations was considered by them sufficiently 

 good so long as enough was obtained for the work 

 on radio-activity to go on. In time it was shown, 

 however, that radio-active substances form com- 

 pounds, and possess chemical properties similar to those 

 of the common elements, and also that these chemical 

 properties can be determined more quickly and even 

 more accurately than those of ordinary bodies. \Mien 

 a complete investigation of the chemical properties of 

 the whole of the radio-active elements was under- 

 taken and completed it was found that they fell 

 into ten groups, the members of each group having 

 identical chemical properties. For these ten groups 

 there were twelve places in the periodic system waiting 

 vacant, so that there was more than enough. The 

 hypothesis of isotopes has made it possible, therefore, 

 to fit in the forty radio-active elements into the ten or 

 twelve vacant places of the periodic system, and in this 

 way the difficulty stated abov'e has been obviated. 



In illustration of the above let us consider the element 

 lead. Lead has an atomic weight of 207*2 ; its atomic 

 number is 82 ; it is not radio-active. It has at least 

 seven isotopic elements. These have the same atomic 

 number as lead, namely 82, exactly the same chemical 

 ' C. G. Darwin, Discovery, vol. i, p. 41. 



