THE CHEMICAL ELEMENTS. 601 



mous number of them all mixed together, and free, as it were, to find 

 their own level. Nor is this all. Astronomers have not only deter- 

 mined that the sun is a star, and have approximately fixed his place in 

 nature as regards size and brilliancy, but they have compared the spec- 

 trum of this star, this sun of ours, with those of the other bodies which 

 people space, and have thus begun to lay the foundations of a science 

 which we may christen Comparative Stellar Chemistry. Dealing with 

 the knowledge already acquired along this line, we may say roughly 

 that there are four kinds of stars recognizable by their spectra. 



We have first the brightest and presumably hottest stars, and of 

 these the spectrum is marvelously simple — so simple, in fact, that we 

 say their atmospheres consist in the main of only two substances — a 

 statement founded on the observation that the lines in the spectra are 

 matched by lines which we see in the spectra of hydrogen and calcium ; 

 there are traces of magnesium, and perhaps of sodium too, but the 

 faintness of the indication of these two latter substances only inten- 

 sifies the unmistakable development of the phenomena by which the 

 existence of the former is indicated. 



So much, then, for the first class ; now for the second. In this we 

 find our sun. In the spectra of stars of this class, the indications of 

 hydrogen are distinctly enfeebled, the evidences by which the existence 

 of calcium has been traced in stars of the first class are increased in 

 intensity, and, accompanying these changes, we find all simplicity 

 vanished from the spectrum. The sodium and magnesium indications 

 have increased, and a spectrum in which the lines obviously visible 

 may be counted on the fingers is replaced by one of terrific complexity. 



The complexity which we meet with in passing from the first class 

 to the second is one brought about by the addition of the lines pro- 

 duced by bodies of chemical substances of moderate atomic weight. 

 The additional complexity observed when we pass from the second 

 stage to the third is brought about by the addition of lines due in 

 the main to bodies of higher atomic weight. And — this is a point of 

 the highest importance — at the third stage the hydrogen, which 

 existed in such abundance in stars of the first class, has now entirely 

 disappeared. 



In the last class of stars to which I have referred, the fourth, the 

 lines have given place to fluted bands, at the same time that the light 

 and color of the star indicate that we have almost reached the stage 

 of extinction. These facts have long been familiar to students of solar 

 and stellar physics. Indeed, in a letter written to M. Dumas, December 

 3, 1873, and printed in the " Comptes Rendus," I thus summarized a 

 memoir which has since appeared in the " Philosophical Transactions" : 



II semble que plus une 6toile est chaude, plus son spectre est simple, et que 

 les elements metaUiques se font voir dans Tordre de leurs poids atomiques.' 

 Ainsi nous avons : 



' The old system of atomic weights was the one referred to. 



VOL. XIT. — 39 



