44 



DISCOVERY 



results is very difficult, and fortunately, when the 

 problem had reached this stage, a less direct but 

 much more powerful method had been developed, which 

 in the course of a very short time finished it off. 



The experiments of Lauc and his collaborators proved 

 that X-rays were a type of light,' and the Braggs dis- 

 covered that there was a characteristic spectrum 

 emitted by the substance of the target of their X-ray 

 tube. Moseley • developed an exceedingly powerful 

 method of finding the characteristic spectrum of any 

 substance, and in the course of a few months examined 

 the majority of the elements. Whereas the visual 

 spectrum of a substance depends on its place in the 

 periodic table (for example, the spectra of lithium, 

 sodium, and potassium all show a strong family re- 

 semblance), in the X-ray spectrum no periodicity is 

 shown, and instead there is a perfectly orderly pro- 

 gression from element to element along the list. The 

 structure of all the spectra is exactly the same, and the 

 only difference is that each in succession has all the 

 wave-lengths of its lines reduced in a definite proportion. 

 Moseley 's work extended throughout the region between 

 aluminium and gold, and it has since been extended 

 up to uranium. More remarkable still is the fact that, 

 if the system is carried back to the beginning of the list, 

 where no X-rays can be found at all, the spectrum cor- 

 responds to the ordinary visual spectrum of hydrogen. 



Now here we have an absolutely certain way of 

 counting elements, and Moseley of course took advan- 

 tage of it. He at once showed that cobalt should come 

 before nickel (this the less direct X-ray methods of 

 Barkla had done already), and a little later that potas- 

 sium comes next to calcium (argop, being gaseous, could 

 not be tested), and that tellurium comes before iodine. 

 He also confirmed the two " manganese " gaps pre- 

 dicted from the periodic table. He then turned his 

 attention to the rare earths, and found that they do 

 not differ from other elements in the least, but show the 

 same orderly progression. His study revealed the 

 extraordinary difficulties which face the rare-earth 

 chemist. Sadts of a rare earth warranted pure would 

 be found to contain 80 per cent, of other rare earths. 

 He examined all the specimens he could collect, and 

 found 14 elements between lanthanum and tantalum, 

 with one gap. Now it happened that at this very time 

 Urbain, the most famous living specialist in this sub- 

 ject, himself the discoverer of several of the rare earths, 

 thought that he had isolated a new one. Moseley's 

 method provided an instant test, and Urbain therefore 

 brought his specimen to Oxford to be examined. It 



• See any modem book on X-rays, for example W. H. and 

 \V. L. Bragg, X-rays and Crystal Structure. 



- H. G. J. Moseley worked in Manchester and afterwards at 

 Oxford. He was killed during the Suvla Bay landing at Galli- 

 poli, serving as Signals Officer to an infantry brigade, an 

 immense loss to science. 



was found to consist of a mixture of three known 

 elements with no trace of the missing one. 



Thus the outcome of Moseley's work is that there 

 are definitely three and no more unknown elements 

 between aluminium and gold. Two are somewhat like 

 manganese, and would have atomic weights about 100 

 and 190. The third is a rare earth with atomic weight 

 about 148. Below aluminium X-ray methods are inapy- 

 plicable,but from the absolute regularity of the periodic 

 table in this region, and above all from the correspon- 

 dence of the X-ray spectra with the visual spectrum of 

 hydrogen, there can be no gaps there. Thus we can 

 \vith certainty number the elements, starting with 

 hydrogen at i and ending with uranium at 92. There 

 are five missing numbers — 43, 61, 75, and in addition 

 two more, 85 and 87; but the Icist are in the radio-active 

 region, and we must turn to another type of evidence. 



We have seen that radio-activity consists in a sub- 

 stance transmuting itself through a succession of 

 states, each of which is a chemical element. The 

 sequences are now well known, and all told something 

 like forty transformations have been found. They 

 all originate from either uranium or thorium, and there 

 has long been very little doubt that they end their 

 radio-active career as lead. The majority are never 

 present in more than infinitesimal amount, but in a 

 few cases they can be isolated and studied by ordinary 

 chemical methods. Thus radium itself resembles 

 barium, and so is naturally assigned a place under it 

 in the periodic table. In spite of their resemblance, 

 the separation of radium from barium is quite an easy 

 process, but this is not so in the case of all radio- 

 elements. Thus there is a substance called Radium D 

 which is present in considerable quantity in uranium 

 ores. It is always separated from the ores along with 

 the lead. As it would be usefiJ to have it as a radio- 

 active source for some purposes, much effort has been 

 expended in trying to purify it from the lead, but 

 always without success. There are many other cases 

 where substances, undoubtedly different from their 

 radio-active behaviour, once mixed together cannot 

 be separated or distinguished chemically or even 

 spectroscopically. 



Now consider how this fits in with our ideas of 

 atomic number. We know of 43 different radio-active 

 elements, but there are only 9 numbers available for 

 them, those from 92 to 84. To reconcile these facts, 

 the hypothesis of "isotopes" was put forward.' Isotopes 

 are substances of different radio-active beha\'iour, but 

 with the same chemical characters, and the hypothesis 

 asserts that such will be quite inseparable by any 

 chemical means. It will be seen that this fits in per- 



' Simultaneously by A. S. Russell, F. Soddy, and K. Fajans. 

 The word "isotope" is due to Soddy, and implies the " same 

 position " in the periodic table. 



