172 THE CHEMISTRY OF THE STARS. 



star from star, must be accurately known before we can obtain the 

 greatest amount of information from our inquiries. 



Our next duty then, clearly, is to study the sun — a star so near us 

 that we can examine the different parts of its atmosphere, which we 

 can not do in the case of the more distant stars. By doing this we 

 may secure facts which will enable us to ascertain in what parts of the 

 atmosphere the absorption takes place which produces the various phe- 

 nomena on which the chemical classification has been based. 



It is obvious that the general spectrum of the sun, like that of stars 

 generally, is built up of all the absorptions which can make themselves 

 felt in every layer of its atmosphere from bottom to top; that is, from 

 the photosphere to the outermost part of the corona. Let me remind 

 you that this spectrum is changeless from year to year. 



Now, sun-spots are disturbances produced in the photosphere; and 

 the chromosphere, with its disturbances, called prominences, lies directly 

 above it. Here, then, we are dealing with tne lowest part of the sun's 

 atmosphere. We find first of all that, in opposition to the changeless 

 general spectrum, great changes occur with the sun-spot period, both in 

 the spots and chromosphere. 



The spot spectrum is indicated, as was found in 1866, by the widening 

 of certain lines; the chromospheric spectrum, as was found in 1868, by 

 the appearance at the sun's limb of certain bright lines. In both cases 

 the lines affected, seen at any one time, are relatively few in number. 



In the spot spectrum, at a sun spot minimum, we find iron lines 

 chiefly affected; at a maximum they are chiefly of unknown or unfa- 

 miliar origin. At the present moment the affected lines are those 

 recorded in the spectra of vanadium and scandium, with others never 

 seen in a laboratory. That we are here far away from terrestrial 

 chemical conditions is evidenced by the fact that there is not a gram 

 of scandium available for laboratory use in the world at the present 

 time. 



Then we have the spectrum of the prominences and the chromosphere. 

 That spectrum we are enabled to observe every day when the sun 

 shines as conveniently as we can observe that of sun spots. The chro- 

 mosphere is full of marvels. At first, when our knowledge of spectra 

 was very much more restricted than now, almost all the lines observed 

 were unknown. In 1868 I saw a line in the yellow, which I found 

 behaved very much like hydrogen, though I could prove that it was not 

 due to hydrogen ; for laboratory use the substance which gave rise to 

 it I called helium. Next year I saw a line in the green at 1474 of 

 Kirchhoff's scale. That was an unknown line, but in some subsequent 

 researches I traced it to iron. From that day to this we have observed 

 a large number of lines. They have gradually been dragged out from 

 the region of the unknown, and many are now recognized as enhanced 

 lines, to which I have already called attention as appearing in the 

 spectra of metals at a very high temperature. 



