156 THE CHEMISTRY OF THE SUN. [CHAP. 



Again, the longest lines of aluminium (wave-lengths 3961 

 and 3943) occur usually in the spectrum of iron as longish 

 lines, and are to be found in the arc spectra of cobalt, nickel, 

 calcium, strontium, barium and other metals, where they are 

 even longer than some of the true lines of the metals in 

 which they occur owing to their lower volatility. 



It will be seen, then, that amongst the first questions raised 

 was that of impurities, and it became clear that if this view 

 were well founded, the revision of all cases of coincidences re- 

 corded by previous observers became absolutely necessary, since 

 at last we had a method of eliminating impurities with certainty. 



3. Light thrown on the Fraurihofer Lines. 



There was another point in which our knowledge was at once 

 increased in the most definite manner. Only those who knew 

 least about solar matters took KirchhofFs statement as to 

 the identity of solar and terrestrial spectra au pied de la lettre ; 

 and Angstrom had already pointed out discrepancies in the 

 case of aluminium and zinc. Hence I attempted to see whether 

 there was anything to help us in the simplification of spectra 

 brought about by passing from the metal to the chloride. It 

 was soon found that when we compared the spectra of metallic 

 vapours reversed in the solar spectrum, such spectra being 

 mapped by the new method, i.e., showing the long and short 

 lines, the reversed lines in the spectrum were invariably those 

 which are longest. Here we had, in fact, a new test to apply 

 to the reversal of solar lines, and a guide of the highest 

 value in spectrum observations of the chromosphere and photo- 

 sphere. It was seen at once that to the last published table 

 of solar elements (that of Thalen) must be added zinc, alu- 

 minium, and possibly strontium, as a result of the application 

 of the new test which it seemed worth while to pursue further. 



In order to continue the inquiry under the best conditions, 

 complete maps of the long and short lines of all the elements 



