CHAPTER XIII. 



DIFFICULTIES. 



AT this point we must pause. The work referred to in the 

 immediately preceding chapters, which brought us down 

 to the end of the year 1873, was chiefly laboratory work. 

 We must now return to the sun, and study not only the 

 application of this new work to solar chemistry, but refer to 

 some purely solar observations as well. 



In this way we shall be able to note how the solar theories 

 then in vogue stood the strain of the new tests we could apply 

 to them ; in short, we can inquire if the line was all clear for 

 future progress. 



We were working on two hypotheses or inferences more 

 or less clearly enunciated by KirchhofF. 



I. The absorption which produced the Fraunhofer lines took 

 place at some distance above the photosphere, the spots being 

 solar clouds. 



II. The chemical elements present in the solar atmosphere 

 were identical with some of those existing on the earth, and 

 their spectra were identical. 



Was the way then perfectly clear, taking the work as it stood 

 in 1873 ? Did work on the sun's atmosphere both during eclipses 

 and independently of them, confirm Kirchhoff's views ? And, 

 moreover, did our chemical theories which Kirchhoff had taken 

 as a base for his second hypothesis explain all the facts which 



M 



