1881.] 



On the Spectra of Sun-spots. 



205 



mass of vapour ultimately to form a star, each reduction of tempera- 

 ture increases the number and complexity of the chemical forms by 

 rendering new combinations possible, these new combinations will con- 

 tain the earlier ones in different proportions. If, for instance, the 

 members of the iron groups are not elementary, they will contain 

 earlier forms, as the salts of calcium contain calcium, which once 

 existed as calcium in the atmosphere of the earth before the salts were 

 produced as the result of a subsequent condensation brought about by 

 cooling. 



The discussion in the accompanying map shows that this is really 

 the case, and that it is the lines which are common to two or more 

 substances which in the main produce the spectra of the lower, and 

 therefore hotter, region of the solar atmosphere. This natural result 

 at once explains the strange variations from the ordinary spectra 

 which have puzzled observers ever since the new method was intro- 

 duced. More than this, these are precisely those lines which have 

 their intensities strengthened when we pass from the arc to the coil. 



It is very instructive to note the gradual simplification of the iron 

 spectrum by increased temperature as we pass from the arc through 

 the spots to the flames, and how with this increasing simplicity we find 

 the basic character of the lines increasing, only one basic line on the 

 map having been yet observed among those lines seen in the arc alone. 



The accompanying figure shows what happens with regard to three 

 adjacent iron lines under different solar and terrestrial conditions, and 

 1 give it because it indicates that one of the most important inquiries 

 to be taken up in the eclipses of next year and 1883, will be the 



