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SCIENCE. 



[N. S. Vol. XVI. No. 407. 



ous, and eleven may be reasonably ascribed 

 to argon, krypton and xenon, one to more 

 volatile gas of the neon class, and the 

 brightest ray of all is but a very little less 

 refrangible than the characteristic auroral 

 ray, and coincides with a strong ray of cal- 

 cium, but also lies between, and close to, an 

 argon and a neon ray, neither of them 

 weak rays. There may be some doubt 

 about the identification of the spectral rays 

 of auroras because of the wide limits of 

 the probable errors in measuring wave- 

 lengths so faint as most of them are, but 

 there is no such doubt about the wave- 

 lengths of the rays in solar protuberances 

 measured by Deslandres and Hale. Stas- 

 sano found that these rays, 44 in number, 

 lying between the Praunhofer line F and 

 3,148 in the ultra-violet agree very closely 

 with rays which Professor Liveing and my- 

 self measured in the spectra of the most 

 volatile atmospheric gases. It will be re- 

 membered that one of the earliest sugges- 

 tions as to the nature of solar prominences 

 was that they were solar auroras. This 

 supposition helped to explain the marvel- 

 lous rapidity of their, changes, and the 

 apparent suspension of brilliant self-lumi- 

 nous clouds at enormous heights above the 

 sun's surface. Now the identification of 

 the rays of their spectra with those of the 

 most volatile gases, which also furnish 

 many of the auroral rays, certainly sup- 

 ports that suggestion. A stronger support, 

 however, seems to be given to it by the 

 results obtained at the total eclipse of May, 

 1901, by the American expedition to Su- 

 matra. In the Astrophysical Journal for 

 June last is a list of 339 lines in the spec- 

 trum of the corona photographed by Hum- 

 phreys, during totality, with a very large 

 concave grating. Of these no fewer than 

 209 do not differ from lines we have meas- 

 ured in the most volatile gases of the atmos- 

 phere, or in krypton or xenon, by more 

 than one unit of wave-length on Arm- 



strong's scale, a quantity within the limit 

 of probable error. Of the remainder, a 

 good many agree to a like degree with ar- 

 gon lines, a very few with oxygen lines, 

 and still fewer with nitrogen lines; the 

 characteristic green auroral ray, which is 

 not in the range of Humphreys' photo- 

 graphs, also agrees within a small fraction 

 of a unit of wave-length with one of the 

 rays emitted by the most volatile atmos- 

 pheric gas. Taking into account the 

 Praunhofer lines H, K and G, usually as- 

 cribed to calcium, there remain only fifty- 

 five lines of the 339 unaccounted for to the 

 degree of probability indicated. Of these 

 considerably more than half are very weak 

 lines which have not depicted themselves 

 on more than one of the six films exposed, 

 and extend but a very short distance into 

 the sun's atmosphere. There are, however, 

 seven which are stronger lines, and reach 

 to a considerable height above the sun's 

 rim, and all have depicted themselves on 

 at least four of the six films. If there be 

 no considerable error in the wave-lengths 

 assigned (and such is not likely to be the 

 case), these lines may perhaps be due to 

 some volatile element which may yet be 

 discovered in our atmosphere. However 

 that may be, the very great number of close 

 coincidences between the auroral rays and 

 those which are emitted under electric ex- 

 citement by gases of our atmosphere al- 

 most constrains us to believe, what is in- 

 deed most probable on other grounds, that 

 the sun's coronal atmosphere is composed 

 of the same substances as the earth's and 

 that it is rendered luminous in the same 

 way— namely, by electric discharges. This 

 conclusion has plainly an important bear- 

 ing on the explanation which should be 

 given of the outburst of new stars and of 

 the extraordinary and rapid changes in 

 their spectra. Moreover, leaving on one 

 side the question whether gases ever be- 

 come luminous by the direct action of 



