TRANSACTIONS OF THE SECTIONS. 6 



compai-atively low temperature forma one of the constituents of the solar atmo- 

 sphere, inasmuch as the dark liuo D in the spectrum of the sun coincides in position 

 with the brijjht line gjven out by incandescent sodium vapour. 



This method of research wtis greatly extended by Kirchhoff, who soon found 

 tliat many of the darlc lines in the solar spectrum were coincident with the bright 

 lines of sundry incandescent metallic 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 the hands of 

 Ilugg-ins, Miller, Secchi, and others, to tlie more distant heavenly bodies. It was 

 speedily found that the fixed stars had constitutions very similar to that of the sun. 

 But a peculiar and unexpected success was attained when some of the nebulfB were 

 examined spectroscopically. To-day it seems (so rapidly has knowledge progi-essed) 

 very much like recalling an old superstition to remind you that, until the advent of 

 the spectroscope, the irresolvable nebulae wei'e considered to be gigantic and remote 

 clusters of stars, the individual members of which vv'ei-e too distant to be separated 

 from each other even with a telescope like that of Lord Rosse. 



But Mr. Iluggins, by means of the spectroscope, soon foimd that this was not the 

 case, and that most of the nebulos which had defied the telescope gave indications 

 of incandescent hydrogen gas. 



It was also found by this observer that the proper motions of some of the fixed 

 stars in a direction to or from the earth might be detected by means of the displace- 

 ment of their spectral lines, a principle of research which was first enunciated by 

 Fizeau. 



Hitherto in such applications of the spectroscope, the body to be examined was 

 viewed as a whole. 



It had not yet been attempted to localize the use of this instrument so as to 

 examine particular districts of the sun — as, for instance, a sun-spot, or the red flames 

 already proved by De La Rue 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 

 spectrum of the darker portion of the spot. 



Br. Frankland had previously found that thick spectral lines correspond to great 

 pressure, and hence the inference from the greater thickness of lines in the umbra 

 of a spot is, that this umbra or dark portion is subject to a greater pressure — that is 

 to say, it exists below a greater depth of the solar atmosphere than the general sur- 

 face of the sun. Thus the results derived from the Kew photoheliograph and those 

 derived from the spectroscope were found to confirm each other. Mr. Lockyer next 

 caused a powerful instrument to be constructed for the purpose of viewing spectro- 

 scopically the red flames round the sun's border, in the hope that if they consisted of 

 ignited gas the spectroscope would disperse, and thus dilute and destroy, the glare 

 which prevents them from being seen on ordinary occasions. 



Before this instrument was quite ready, these flames had been analyzed spec- 

 troscopically by Capt. Herschel, M. Jansseu, and others on the occasion of a total 

 eclipse occurring in India, and they were found to consist of incandescent 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 reappeared, from which he argued that a solar eclipse 

 is not necessary for the examination of this region. 



Before information of the discovery made by M. Janssen had reached tliis coun- 

 try, the instrument of Mr. Lockyer had been completed, and he also found that by 

 its means he was able to analyze at leisure the composition of the red flames with- 

 out the necessit}' 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 frequently 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. 



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