27G THE CHEMISTRY OF THE SUN. [CHAP. 



colourless because their absorption is beyond the red, with or 

 without an absorption in the ultra-violet. Glass is a good case in 

 point ; others will suggest themselves as opposed to the opacity of 

 the metals." 



The point in the new experiments, then, to which reference 

 has now to be made was to determine what spectrum the 

 vapours of the metallic bodies gave at a lower temperature 

 than any at which they had been previously observed. It is 

 evidently therefore a question of absorption phenomena. 



The first experiments were made, in December, 1873, upon 

 zinc in a glass tube closed at each end with glass plates ; they 

 were made by myself in conjunction with my friend, Dr. Russell. 

 We studied the absorption of the light of the positive pole of 

 the electric lamp placed at one end of the tube by the vapour 

 in the tube. A one-prism spectroscope was used for the ob- 

 servations. A stream of pure hydrogen was allowed to pass 

 through the tube, which was heated in a Hoffmann's gas 

 furnace. This plan, however, would not work for several 

 reasons among them, the glass melted. 



In the next attempt we used an iron tube some four feet long, 

 closed with glass at the ends as before, with its central part in a 

 charcoal furnace. The air in the tube was replaced by hydrogen 

 and the substance of which the low temperature absorption 

 spectrum was to be examined was dropped, by a special arrange- 

 ment, into the central portion. There was an electric lamp at 

 one end of the tube and a spectroscope at the other, as in the 

 first arrangement. The object of keeping the voltaic arc out- 

 side the tube was of course to enable us to deal with the 

 vapour at the temperature of the furnace merely. In most 

 cases, when the reduction of the density of the vapour enabled 

 any light to be seen, the light was green, and the spectroscope 

 indicated that this was due to the absorption of the red and 

 blue light. Various experiments indicated that the blue and 



