222 



NATURE 



[November 6, 1919 



elements, which were confirmed with astonishing 

 exactitude in the cases of scandium, gallium, and 

 germanium. 



Another factor which played an important rSle in 

 the development of the chemistry of the elements 

 in the early years, of this period was the applica- 

 tion of the spectroscope by Bunsen and Kirchhoff 

 to chemical analysis, when, by a comparison of 

 the bright lines in the spectra of the vapours of 

 metallic elements with the dark lines in the solar 

 spectrum, they showed that many terrestrial 

 elements exist in the sun. During the last two 

 decades the interest in spectroscopy has revived, 

 and much of the valuable information which we 

 now possess of the intra-atomic structures of the 

 elements is due to the remarkable developments 

 in the construction of diffraction gratings, and in 

 particular the concave gratings of Rowland^. 



Notwithstanding the great possibilities for 

 research opened up by Mendeleeff 's periodic table, 

 the latter remained only slightly modified until 

 1893, when a period of rapid development and 

 continual progress began. The later discoveries 

 with regard to the chemical elements fall in a 

 remarkable way into three distinct groups : the 

 rare earths, the inactive gases, and the radio- 

 active elements, and it is to be lamented that the 

 pioneers in the two first-named groups have 

 passed away. 



Much of our knowledge of the rare earths is 

 due to the late Sir William Crookes, who was 

 the first to advance the conception of the meta- 

 elements — i.e. elements which show great resem- 

 blance to each other, and have many physical and 

 chemical properties in common, and, in conse- 

 quence, are not easy to separate. Such in a few 

 words sums up the chief characteristics of the rare 

 earths, which have found so far only a temporary 

 resting place in the periodic table. Apart from 

 their purely academic interest and the high degree 

 of accuracy attained in -their separation, the rare 

 earths have found important technical application 

 as catalytic agents and in the manufacture of the 

 modern incandescent mantle. Our knowledge of 

 them, however, remains in many respects incom- 

 plete. 



Of the second group, the inactive gases, we 

 possess a more complete history of their 

 chemistry, due in no small measure to the brilliant 

 achievements of Lord Rayleigh and Sir William 

 Ramsay, who were the first (1894) to characterise 

 the inert gas argon in the atmosphere, and so 

 confirmed the almost forgotten work of Caven- 

 dish more than a century before. The discovery 

 of helium in cleveite by Ramsay followed shortly 

 after that of argon ; his attention had been 

 directed by Miers to Hillebrand's discovery of 

 nitrogen in the mineral uraninite — and gas-con- 

 taining minerals seemed to be a possible store- 

 house of condensed argon. He sought for argon 

 and found helium, the presence of which in the 

 sun's atmosphere had been detected by Lockyer 

 twenty-five years before. 



The proof that helium was an inert monatomic 

 gas like argon led to many speculations as to 

 NO. 2610, VOL. 104] 



the position of these new elements in the periodic 

 system. Ramsay predicted the existence of 

 another inert gas between, and forming a " triad " 

 with, helium and argon, having an atomic weight 

 between that of fluorine (19) and that of 

 sodium (23), and he and his fellow-workers 

 deliberately hunted for the missing element. They 

 found it in the atmosphere, but besides the gas 

 they sought — neon (20) — they also isolated the 

 heavier elements krypton and xenon. All the 

 inactive gases are colourless ; they form no 

 chemical compounds, and are monatomic. They 

 have definite boiling points, give characteristic 

 Geissler-tube spectra, and occupy a unique posi- 

 tion in the periodic table — the neutral points in 

 Crookes's descending figure of eight. 



The last group of elements to be discovered 

 include the remarkable and interesting series of 

 radioactive elements, which originated in the dis- 

 covery of radium by Mme. Curie in 1898. The 

 development of this field of research has produced 

 a profound effect upon chemical theory and given 

 us entirely new conceptions of the structure and 

 nature of the atom, foremost among which is the 

 nuclear atom proposed by Sir E. Rutherford, and 

 recently modified by Prof. Bohr. 



The chief interest of the radioactive elements 

 centres round two elements of highest atomic 

 weights, uranium and thorium, which are con- 

 tinually decomposing into a series of other 

 elements at definite rates over which we have at 

 present no control. These new elements in a 

 similar way undergo spontaneous changes into 

 still another series of elements. Accompanying 

 these changes in both cases there is a high-speed 

 emission of three distinct kinds of rays, now 

 designated the o-, /3-, and 7-rays respec- 

 tively. The first-mentioned have been identi- 

 fied as electrically charged atoms of helium, and 

 it is now believed that all radio-elements are built 

 up of lead and helium, a conclusion reached by 

 Rutherford and others, and thus after the lapse 

 of a century the hypothesis advanced by Prout 

 (1815), concerning the existence of a primordial 

 substance, makes a reappearance in modern guise. 



The 'majority of the elements formed in the 

 transformations associated with uranium and 

 thorium (which are the progenitors of a long line 

 of descendants) have not as yet been obtained in 

 a pure condition, and are characterised at the 

 present time solely in connection with radioactive 

 properties. Two substances, radium and niton — 

 the gaseous emanation from radium — have been 

 definitely described, and their atomic weights and 

 positions in the periodic table fixed. Niton 

 belongs also to the group of inactive gases ; its 

 existence is transitory, since the gas disappears 

 after a few days, during the course of which 

 radioactive disintegration takes place. Its atomic 

 weight being 222, four units less than radium, 

 the difference is attributed to the loss of a helium 

 atom from radium. 



Based on a consideration of their researches, 

 Rutherford and Soddy have formulated a theory 

 of atomic disintegration (1902) in connection with 



