NATURE 



[August 26, 1920 



in appropriate sequence in the form of books, 

 among which are "The Chemistry of the Sun" 

 (1887), "The Meteoritic Hypothesis" (1890), and 

 "Inorganic Evolution" (1900). His observations 

 and his views on their significance were thus made 

 widely known, and the trend of his work could 

 be the more readily followed. It was especially 

 his desire to impress upon chemists and physicists 

 the importance of the sun and stars as a means of 

 investigating the behaviour of matter at high 

 temperatures, and as possibly throwing light upon 

 the nature of atoms and molecules. 



Among the researches which have had the most 

 potent influence, and have led to very definite 

 advances, were those which dealt with the changes 

 in the spectrum of the same element under dif- 

 ferent conditions of experiment. Lockyer was 

 early led by his solar observations to a compara- 

 tive study of the flame, arc, and spark spectra 

 of some of the metallic elements, and one of his 

 first successes was to show that some of the lines 

 most characteristic of solar prominences, other 

 than those of hydrogen and helium, were produced 

 only under high temperature conditions, while 

 some of those prominently affected in sun-spots 

 were produced at a low temperature. With these 

 and other observations as a basis he put forward, 

 in 1873, his well-known dissociation hypothesis, 

 which became the subject of much Siscussion. 

 The hypothesis supposed that at successively 

 higher temperatures the " molecular groupings " 

 which existed at lower stages were broken up 

 into finer forms of matter, or possibly into new 

 elements, producing different spectral lines, and 

 on this view it was shown that a multitude of solar 

 observations which had seemed to be wholly inex- 

 plicable on the ground of previous laboratory ex- 

 perience became easy of explanation. Thus his 

 view of the construction of the solar atmosphere 

 was that if we could observe a section of it we 

 should see it divided into a number of layers, each 

 with its appropriate spectrum, and the spectrum 

 would be simpler the nearer the layer was to the 

 photosphere. The metallic elements, instead of 

 existing as such in a reversing layer, were con- 

 sidered to be entirely broken up in the vicinity of 

 the photosphere, and their germs distributed 

 throughout the atmosphere, the molecular group- 

 ings becoming more complex as they became 

 further removed from the source of heat. The 

 theory doubtless calls for some re-statement in the 

 light of modern views as to the structure of atoms 

 and the origin of spectra, but it was a valuable 

 guide to observation, and Lockyer anticipated the 

 conclusion reached by St. John in recent years, 

 that the complete absorption of any one element 

 in the solar spectrum is the integration of lines 

 special to various levels in the solar atmosphere. 

 Lockyer himself seems to have been convinced 

 that the ultimate products of dissociation were 

 hydrogen and helium ; but although this is so 

 closely in accord with recent work on the structure 

 of atomic nuclei, it does not seem probable that 

 the phenomena studied by Lockyer were directly 

 NO. 2652, VOL. 105] 



related to those investigated by Rutherford. The 

 writer well remembers numerous attempts to pro- 

 duce the lines of hydrogen or of helium by the 

 passage of powerful condensed discharges between 

 metallic electrodes, all of which, however, were 

 unsuccessful. 



Work on the varying spectra of the elements 

 was vigorously resumed by Lockyer in connection 

 with the interpretation of the photographs of the 

 chromospheric spectrum which had been taken 

 under his direction during the solar eclipses of 

 1893 and 1896, and of a series of photographs of 

 stellar spectra which he had commenced at Ken- 

 sington about 1890. Several elements were in- 

 vestigated over a long range of spectrum, and 

 numerous additional lines were found to be in- 

 tensified on passing from the arc to the spark 

 spectrurti, or which only appeared in the spark. 

 These were designated "enhanced lines," and the 

 work at once led to the definite assignment of 

 origins to many chromospheric and stellar lines 

 which had previously resisted explanation. But 

 this was not all ; the enhanced lines were shown 

 to belong to a special class which were only fully 

 developed at high temperatures, so that they gave 

 valuable evidence of physical conditions in the 

 atmospheres of the sun and stars as well as of 

 their chemical constitutions. It would scarcely 

 be too much to claim that this further work on 

 enhanced lines introduced a new principle into 

 astronomical spectroscopy, inasmuch as it justi- 

 fied the chemical identification of celestial spectra 

 which could not be completely reproduced in the 

 laboratory. The only assumption it was necessary 

 to make was that the series of changes indicated 

 in the flame, arc, and spark would be continued 

 if still more powerful means of excitation were 

 available, so that at sufficiently high temperatures 

 the enhanced lines would be the sole survivors. 

 In accordance with his views on dissociation, and 

 for convenience of reference, the enhanced lines 

 were designated " proto-metallic " lines, and 

 attributed to "proto-metals," which were re- 

 garded as simplified forms of the vapours which 

 yielded the arc lines. 



Apart from any special views as to the cause of 

 their appearance, however, the discovery of en- 

 hanced lines has proved to be of the first import- 

 ance in astrophysical inquiries, and the tables of 

 such lines which were compiled at South Kensing- 

 ton have been much utilised by astronomers 

 throughout the world. Among other applications, 

 as Lockyer was the first to show, the interpreta- 

 tion of the spectra of new stars in their early 

 stages is almost entirely dependent upon a know- 

 ledge of the enhanced lines of iron, titanium, and 

 other elements. In collaboration with his assist- 

 ants, Lockyer showed later that enhanced lines 

 were also developed under the action of strong 

 electrical discharges in non-metallic elements, in- 

 cluding silicon, carbon, sulphur, and nitrogen, 

 and the lines observed in these experiments have 

 also led to important celestial identifications. 

 There can be little doubt that the continuation of 



