August 20, 1891] 



NATURE 



375 



actually into two. The comparison of the results of this inde- 

 pendent analytical method with the remarkable but different 

 conclusions to which M. Lecoq de Boisbaudran and Mr. Crojkes 

 have been led respectively, from spectroscopic observation of 

 these bodies when glowing under molecular bombardment in a 

 vacuum tube, will be awaited with much interest. It is worthy 

 of remark that, as our knowledge of the spectrum of hydrogen in 

 its complete form came to us from the stars, it is now from the 

 sun that chemistry is probably about to be enriched by the dis- 

 covery of new elements. 



In a discussion in the Bakerian Lecture for 1885 of what we 

 knew up to that time of the sun's corona, I was led to the con- 

 clusion that the corona is essentially a phenomenon similar in 

 the cause of its formation to the tails of comets — namely, that it 

 consists for the most part probably of matter going from the sun 

 under the action of a force, possibly electrical, which varies as 

 1 he surface, and can therefore in the case of highly attenuated 

 matter easily master the force of gravity even near the sun. 

 Though many of the coronal particles may return to the sun, 

 those which form the long rays or streamers do not return ; they 

 separate and soon become too diffused to be any longer visible, 

 and may well go to furnish the matter of the zodiacal light, which 

 otherwise has not received a satisfactory explanation. And 

 further, if such a force exist at the sun, the changes of terrestrial 

 magnetism may be due to direct electric action, as the earth 

 moves through lines of inductive force. 



These conclusions appear to be in accordance broadly with 

 the lines along which thought has been directed by the results of 

 subsequent eclipses. Prof. Schuster takes an essentially similar 

 view, and suggests that there may be a direct electric connection 

 between the sun and the planets. He asks further whether the 

 sun may not act like a magnet in consequence of its revolution 

 about its axis. Prof. Bigelow has recently treated the coronal 

 forms by the theory of spherical harmonics, on the supposition 

 that we see phenomena similar to those of free electricity, the 

 rays being lines of force, and the coronal matter discharged from 

 the sun, or at least arranj^^ed or controlled by these forces. At 

 the extremities of the streams for some reasons the repulsive 

 power may be lost, and gravitation set in, bringing the matter 

 back to the sun. The matter which does leave the sun is per- 

 sistently transported to the equatorial plane of the corona ; in 

 fact, the zodiacal light may be the accumulation at great dis- 

 tances from the sun along this equator of such like material. 

 Photographs on a larger scale will be desirable for the full 

 development of the conclusions which may follow from this study 

 of the curved forms of the coronal structure. Prof. Schaeberle, 

 however, considers that the coronal phenomena may be satisfac- 

 torily accounted for on the supposition that the corona is formed 

 of streams of matter ejected mainly from the spot zones with 

 great initial velocities, but smaller than 382 miles per second. 

 Further that the different types of the corona are due to the 

 effects of perspective on the streams from the earth's place at the 

 lime relatively to the plane of the solar equator. 



Of the physical and the chemical nature of the coronal matter 

 we know very little. Schuster concludes, from an examination 

 of- the eclipses of 1882, 1883, and 1886, that the continuous 

 spectrum of the corona has the maximum of actinic intensity dis- 

 placed considerably towards the red when compared with the 

 spectrum of the sun, which shows that it can only be due in 

 small part to solar light scattered by small particles. The lines 

 of calcium and of hydrogen do not appear to form part of the 

 normal spectrum of the corona. The green coronal line has no 

 known representative in terrestrial substances, nor has Schuster 

 been able to recognize any of our elements in the other lines of 

 the corona. 



The spectra of the stars are almost infinitely diversified, yet 

 they can be arranged with some exceptions in a series in which 

 the adjacent spectra, especially in the photographic region, are 

 scarcely distinguishable, passing from the bluish-white stars like 

 Sirius, through stars more or less solar in character, to stars with 

 banded spectra, which divide themselves into two apparently 

 independent groups, according as the stronger edge of the bands 

 is towards the red or the blue. In such an arrangement the 

 sun's place is towards the middle of the series. 



At present a difference of opinion exists as to the direction in 

 the series in which evolution is proceeding, whether by further 

 condensation white stars pass into the orange and red stages, or 

 whether these more coloured stars are younger and will become 

 white by increasing age. The latter view was suggested by 

 Johnstone Stoncy in 18^7. 



NO. II 38, VOL. 44] 



About ten years ago Ritter in a series of papers discussed 

 the behaviour of gaseous masses during condensation, and the 

 probable resulting constitution of the heavenly bodies. Accord- 

 ing to him, a star passes through the orange and red stages 

 twice : first during a comparatively short period of increasing 

 temperature, which culminates in the white stage, and a second 

 time during a more prolonged stage of gradual cooling. He 

 suggested that the two groups of banded stars may correspond 

 to these different periods : the young stars being those in which 

 the stronger edge of the dark band is towards the blue, the other 

 banded stars, which are relatively less luminous and few in 

 number, being those which are approaching extinction through 

 age. 



Recently a similar evolutional order has been suggested, which 

 is based upon the hypothesis that the nebulae and stars consist 

 of colliding meteoric stones in different stages of condensation. 



More recently the view has been put forward that the diversi- 

 fied spectra of the stars do not represent the stages of an 

 evolutional progress, but are due for the most part to differences 

 of original constitution. 



The few minutes which can be given to this part of the 

 address are insufficient for a discussion of these different views, 

 I purpose, therefore, to state briefly, and with reserve, as the 

 subject is obscure, some of the considerations from the characters 

 of their spectra which appeared to me to be in favour of the 

 evolutional order in which I arranged the stars from their photo- 

 trraphic spectra in 1879. This order is essentially the same as 

 Vogel had previously proposed in his classification of the stars 

 in 1874, in which the white stars, which are most numerous, 

 represent the early adult and most persistent stage of stellar life ; 

 the solar condition that of full maturity and of commencing age ; 

 while in the orange and red stars with banded spectra we see the 

 setting in and advance of old age. But this statement must be 

 taken broadly, and not as asserting that all stars, however 

 different in mass and possibly to some small extent in original 

 constitution, exhibit one invariable succession of spectra. 



In the spectra of the white stars the dark metallic lines are 

 relatively inconspicuous, and occasionally absent, at the same 

 time that the dark lines of hydrogen are usually strong, and more 

 or less broad, upon a continuous spectrum, which is remarkable 

 for its brilliancy at the blue end. In some of these stars the 

 hydrogen and some other lines are bright, and sometimes 

 variable. 



As the greater or less prominence of the hydrogen lines, dark 

 or bright, is characteristic of the white stars as a class, and 

 diminishes gradually with the incoming and increase in strength 

 of the other lines, we are probably justified in regarding it as 

 due to some conditions which occur naturally during the pro- 

 gress of stellar life, and not to a peculiarity of original consti- 

 tution. 



To produce a strong absorption-spectrum a substance must be 

 at the particular temperature at which it is notably absorptive ; 

 and, further, this temperature must be sufficiently below that of 

 the region behind from which the light comes for the gas to 

 appear, so far as its special rays are concerned, as darkness upon 

 it. Considering the high temperature to which hydrogen must 

 be raised before it can show its characteristic emission and ab- 

 sorption, we shall probably be right in attributing the relative 

 feebleness or absence of the other lines, not to the paucity of the 

 metallic vapours, but rather to their being so hot relatively to the 

 substances behind them as to show feebly, if at all, by reversion. 

 Such a state of things would more probably be found, it seems 

 to me, in conditions anterior to the solar stage, A considerable 

 cooling of the sun would probably give ri^e to banded spectra 

 due to compounds, or to more complex molecules, which might 

 form near the condensing points of the vapours. 



The sun and stars are generally regarded as consisting of glow- 

 ing vapours surrounded by a photosphere where condensation is 

 taking place, the temperature of the photospheric layer from 

 which the greater part of the radiation comes being constantly 

 renewed from the hotter matter within. 



At the surface the convection currents would be strong, pro- 

 ducing a considerable commotion, by which the different gases 

 would be mixed and not allowed to retain the inequality of pro- 

 poriions at different levels due to their vapour densities. 



Now the conditions of the radiating photosphere and those of 

 the gases above it, on which the character of the spectrum of a 

 star depends, will be determined, not alone by temperature, but 

 also by the force of gravity in these regions ; this force will be 

 fixed by the star's mass and its stage of condensation, and will 

 become greater as the star continues to condense. 



