A?i^: 26, 1875] 



NATURE 



347 



likewise led to'associate the low ^temperature of the bottom of 

 sun-spots with the downward carriage of colder matter from the 

 atmosphere of the sun, while/the upward rush of heated matter 

 was suppj^ei to account f )r the facula; or bright patches which 

 almost invariably accompany spots. In the next place the Kew 

 observers, miking use not only of the Kew series but of those of 

 Schwal)e and Carrington, which were generously placed at their 

 disposal, have discovered trices of the influence of the nearer 

 plan-ts upon the behaviour of sun-spot'. This influence appears 

 1 1 be of such a nature that spo's attain their maximum size when 

 carried by rotation into positions as far as poss'ble remote from 

 the intliencing planet — that is to say into positions where the 

 body of the sun is between them and the planet. There is also 

 evidence of an excess of solar action when two influential planets 

 come near together, liut although considerable light has thus 

 b»en thrown on the periodicity of sun-s lof, it ought to be borne 

 in mind that the cius» of the remarkable period of eleven years 

 and a quarter, originally discovered by Schwabe, has not yet been 

 properly explained. The Kew observers have likewi'ie dis- 

 covered trices of a peculiar osdllation of spots between the two 

 hemis oheres of the sun, and finally their researches will p'ace at 

 the command of the observers the da^a fo' ascertaining whether 

 cenTis of g-eater and lesser solar activity are connected with 

 cer'ain heliocentric positions. 



Wtiile the sun's surface was thus being examined both tele- 

 scopically and photographically, the spectro'icope cime to be 

 empl )yed as an instmment of research. It had already been 

 surmised by Prof Stokes, that the vapour of sodium at a com- 

 paratively low temperature forms one of the constituents of the 

 sohr atmosphere, inasmuch as the dirk line D in the spectrum 

 of the sun coinci fes in positioi with the bright line given out by 

 incandescent sodium vapour. 



Tnis method of research wa^ greatly eictended by KirchhofT, 

 ■who soon found that many of the dark lines in the solar spec- 

 trum were coincident with the bright lines of sundry incandescent 

 metillic vapours, and a good beginning was thus made towards 

 ascertaining the chemical constitution of the sun. 



The new method soon brought forth further fruit when applied 

 in th^ hands of lluggins. Miller, Secchi, and others, to the more 

 di tant heavenly bodies. It was speedily found that the fixed 

 star, had constitutions very sitnilar to that of the sun. But a 

 pe.'uliar and unexpected success was attained when some of the 

 nebulae were examined spectrojcopically. Today it seems (so 

 rapidly has knowledge progress;;d) very much like recalling an old 

 s ipe stition to remind you that untd the advent of the s^pectro- 

 Fcope the irresolvable nibulrc were considered to be gigmtic and 

 rem Jte clusters of stars, the individual members of which were 

 too distant to b^ sepira'ed from each other even with a telescope 

 like that of Lord Rosse. But Mr. lluggins, by mean; of the 

 spectroscope, soon found that this was not the case, and that 

 most of the nebuUv which had defied the telescope gave indica- 

 tions of incandescent hydrogen gas. It was also fjuiid by this 

 observer that the proper motions of some of the fixed s'.ars in a 

 direction to of from the earth mi^ht be detected by means of the 

 displa:ement o' their spectral lines, a method of research which 

 was first eaunciated by Fizeau. Hitherto in such ajiplicaions 

 of the spectroscope, the body to be txainincd was viewed as a 

 whole. It had not )et been a'.tempitd lodocalise the use of this 

 instrument so as to examine paiticular districts of the tun, as for 

 iiistince a sun-spot, or the red flames already proved by De la 

 Kue to belong to our luminary. This application was first made 

 by Mr. Lockyer, who in the year 1865 examined a sun-spot 

 spectroscopically and remarked the greater thickness of the lines 

 in the s^'ectrum of the darker portion of the spot. 



Dr. Frankland had previously found that thick spectral lines 

 correspond tj great pressure, and hence thi inference from the 

 }.rjater thickness of lints in the umbra of a spot is that this 

 umbra or dark poriiou is subject to a greater pressure ; tiial is to 

 s .y, it exijti beloA' a greater depth of the solar atmosphere than 

 tie general surface of the sun. Thus the results derived from 

 the Kew photoheliOjjraph and those derived from the spectroscope 

 Were found to confir.n each other. Mr. Lockyer next caused a 

 J oAtriul instrument to be constructed for the purpose of viewing 

 s-i)tclroscoi)icaily the red flames round the sun's border, in the 

 lijpe that if they consisted of ignited gas the spectroscope would 

 disperse, and thus dilu.e and destroy the glare which prevents 

 them from being seen oa ordinary occasion.^. 



^ Before this instrument was quite ready tl ^se flames had been 

 ahaljsed spectroscopically by Capt. IIers,Iitl, M. Janssen, and 

 others, on the occasion of a total eclipse occurring in India, and 

 they were found to consist of incandr ^qvX gas, most probably 



hydrogen. But the latter of these observers (M. Janssen) made 

 the important observation that the bright lines in the spectrum 

 of these flames remained visible even after the sun had reap- 

 peared, from which he argued that a solar eclipse is not neces- 

 sary for the examination of this region. 



Before information of the discovery made by Janssen had 

 reached this country, the instrument of Mr. Lockyer had been 

 completed, and he also found that by its means he was able to 

 analyse at leisure the composition of the red flames without the 

 necessity of a total eclipse. An atmosphere of incandescent 

 hydrogen was found to surround our luminary into which, during 

 the greater solar storms, sundry metallic vapours were injected, 

 sodium, magnesium, and iron forming the three that most fre- 

 quently made their appearance. 



Here we come to an interesting chemical question. 

 It had been remarked by Maxwell and by Pierce as the result 

 of the molecular theory of gases that the final distribution of 

 any number of kinds of gas in a vertical direction under gravity 

 is such that the density of each gas at a given height is the same 

 as if all the other gases had been removed, leaving it alone. 

 In our own atmosphere the continual disturbances prevent this 

 arrangement from taking place, but in the sun's enormously 

 extended atmosphere (if indeed our luminary be not nearly all 

 gaseous) it appears to hold, inasmuch as the upper portion of 

 this atmosphere, dealing with known elements, apparently con- 

 sists entirely of hydrogen. A^arious other vapours are, how- 

 ever, as we have seen, injected fr<im below the photosphere into 

 the solar atmosphere on the occasion of great oisturbances, and 

 Mr. Lockyer has a^ked the question, whether we have not here 

 a true indication of the relative densities of these various vapours 

 derived from the islative heights to which they are injected on 

 such occasions. 



This qu°stion has been asked, but it has not yet received a 

 definite solution, for chemists tell us that the vapour densities of 

 some of the gases injected into the sun's atmosphere on the 

 occasion of disturbances are, as far as they know from terrestrial 

 observations, different from those which would be indicated by 

 taking the relative heights attained in the atmosphere of the sun. 

 Mr. Lockyer Kas attempted to bring this question a step nearer 

 to its solution by showing that the vapours at the temperatures 

 at which their vapour densities have been experimentally deter- 

 mined are not of similar molecular constitution, whereas in the 

 sun we get an indication, from the fact that all the elements give 

 us line spectra, that they are in similar molecular states. 



Without, however, attempting to settle this question, I may 

 remark that we have here an interesting example of how two 

 branches of science — physics and chemistry— meet together in 

 solar research. 



It had already been observed by Kirchhoff that sometimes one 

 or more of the spectral lines of an elementary vapour appeared 

 to be reversed in the solar spectrum, while the other lines did 

 not experience reversal. Mr. Lockyer succeeded in obtaining 

 an explanation of this phenomenon. This explanation was found 

 by means of the method of localisation already mentioned. 



Hitherto, when taking the spectrum of the electric spark be- 

 tween the two metallic poles of a coil, the arrangements were 

 such as to give an average spectrum of the metal of these poles ; 

 but it was found that when the method of localisation was 

 employed, different portions of the spark gave a different number 

 of lines, the regions near the terminals being rich in lines, while 

 the midway regions give comparatively few. 



If we imagine that in the midway regions the metallic vapour 

 given off by the spark is in e. larer slate than that near the poles, 

 we are thus led to regard the short lines which cling to the poles 

 as those which require a greater density or nearness of the vapour 

 ])articles before they make their appearance ; while on the other 

 hand, those which extend all the way between the two poles 

 come to be regarded as those which will continue to make their 

 appearance in vapour of great tenuity. 



Now it was remarked that these long lines were thtvery lines 

 which were reversed in the atmosphere of the sun. Hence when 

 we observe a single coincidence between a dark solar line and 

 the baght line ofany metal, we arc further led to inquire whether 

 tliis bright line is one of the long lines which will continue to 

 exist all the way between two terminals of that metal when the 

 spark passes. 



If this Le the case, then we may argue with much probability 

 that the metal in question really occurs in the solar atmosphere ; 

 but if, on the other hand, the coincidence is merely between a 

 solar dark line and a short bright one, then we are led to imagine 

 that it is not a true coincidence, but something which will 



