On the Chemistry of the Hottest Stan 



155 



This method of looking for the longest lines will fail in the case of 

 stars which are hotter than our hottest spark. In such case, there- 

 fore, we must necessarily rely on a comparison with lines which, from 

 our study of the spectra at different temperatures, would most pro- 

 bably be longest in the spectrum at a temperature higher than any at 

 which experiments can be carried on. 



It is in connection with such an inquiry as this that the study of 

 the conditions of the sun's atmosphere is of supreme importance. It 

 is obvious that a knowledge of them must be of the utmost value in 

 enabling us to apply a well-established series of facts, gathered in the 

 case of the star nearest to us, to the phenomena presented by the 

 more distant bodies. 



Observations of eclipses, especially those of 1882 and 1893, provide 

 us with a series of results with which to approach the question of the 

 absorption phenomena presented by the stars, and we have seen that 

 the whole question of the classification of stars depends absolutely 

 upon their absorption phenomena. 



Now in many of these bodies the atmosphere may be millions of 

 miles high. In each star the chemical substances in the hottest and 

 coolest portion may be vastly different ; the region, therefore, in 

 which the absorption takes place, which, spectroscopically, enables us 

 to discriminate star from star, must be accurately known before we 

 can obtain the greatest amount of information from our inquiries. 



The loci of the various absorptions in a mass of gas such as a star 

 of the highest temperature, have not yet been sufficiently studied. 



I may say that for some time I was of opinion that in the sun 

 many of the darkest lines indicated absorptions high up in the atmo- 

 sphere, for the reason that the bright continuous spectrum of the 

 lower levels might have an important effect upon line absorption 

 phenomena by superposing radiation, and so diminishing the initial 

 absorption. The observations of the eclipse of 1893, however, indi- 

 cate that this opinion is probably only strictly true when the strata 

 of the sun's atmosphere close above the photosphere are considered. 



If we are justified in arguing from a star with a photosphere as 

 well developed as that of the sun, to one in which it is in all prob- 

 ability much less marked, in consequence of a much higher tempera- 

 ture, then we must consider that the absorptions which mark out the 

 various star groups are more conditioned by the temperatures of the 

 hottest regions merely than by the thickness of the absorbing atmo- 

 spheres, or by the densities of the various vapours. Another con- 

 sideration to be borne in mind is that if the atmospheres are in part 

 composed of condensable vapours, and not entirely of gases permanent 

 at all stellar temperatures, condensation must always be going on 

 outside at the region of lowest temperature. 



At this point it is important to consider the conditions of that part 



