No. 7. J WURTZ — THEORY OF ATOMS. 391 



The linei of Fraunhofer are dark, only the lines of the protu- 

 berances and those seen a moment after the disappearance of the 

 Bun in an eclispe, and a moment before its reappearance, are 

 bright, like those which characterise the spectra of incandescent 

 metallic vapours. Here we have a curious relationship which 

 has furnished most important and precise indications on the 

 physical constitution of the sun. 



I have spoken of the chemistry of the sun, but the spectro- 

 scope has explored all the far-off space of heaven. The light of 

 hundreds of stars has been analysed, and nebulae, scarcely visible, 

 Jiave had the quality of their radiations revealed by its aid. 

 The light, in some cases very feeble, with which a number of 

 stars shine, gives a spectrum with dark lines like the solar 

 spectrum, and this fact proves to us that the constitution of 

 these stars is like that of our sun. Aldebaran sends us records 

 of hydrogen, magnesium, and calcium, which abound in solar 

 light, but also those of metals which are rare or absent, as 

 tellurium, antimony, and mercury. 



Nebulae, twenty thousand times less brilliant than a candle at 

 a distance of 400 metres, have still given a spectrum, for their 

 light, although feeble, is very simple in its constitution, and the 

 spectrum which it gives consists only of two or three bright 

 bands, one of hydrogen, the other of nitrogen. These nebulas 

 which give a spectrum of bright lines, are those which the most 

 powerful telescopes cannot resolve : there is an "abyss" between 

 them and resolveable nebulae, which, like ordinary stars, give a 

 spectrum with dark lines. 



What an effort of the human mind ! To discover the consti- 

 tution of stars of which the distances even are unknown ; of 

 nebulae which are not yet worlds ; to establish a classification of 

 all the stars, and still more to guess their ages — ah, tell me, is not 

 this a triumph for science ? Yes, we have classed them according 

 to their ages. Stars coloured, stars yellow, stars white; the 

 white are the hottest and the youngest; their spectrum is 

 composed of a few lines only, and these lines are dark. 

 Hydrogen predominates. Traces of magnesium are also met 

 with, of iron, and perhaps of sodium, and if it is true that 

 Sirius was a red star in the time of the ancients, it owed per- 

 haps its tint to the greater abundance of hydrogen at that epoch. 

 Our sun, Aldebaran, Arcturus, are among the yellow stars. In 

 their spectra the hydrogen lines are less developed, but the 



