388 THE CHEMISTRY OF THE SUN. [CHAP. 



3. Experiments with the Shielded Arc. 



I referred on page 382 to the general conditions which 

 prevent the full development of absorption phenomena when 

 a naked arc is employed. 



From what has previously been said, it will be clear that 

 before we can obtain the reversal of any spectral line, which is 

 due to the vibration of a particular molecule, we must interpose 

 between the spectroscope and that molecule a similarly vibrating 

 molecule at lower temperature. 



A molecule near the centre of the arc will give bright lines 

 because it is so near the highest temperature ; and in the naked 

 arc, absorption may not be observed because the similar mole- 

 cules outside it are instantly cooled or oxidised, unless it happen 

 to be a molecule given off in such large quantity with the tem- 

 perature employed, that these causes of their disappearance are 

 overpowered. 



It is obvious that the arc can be shielded from the effects 

 above referred to either by raising the temperature of the sur- 

 rounding atmosphere or making that atmosphere consist of 

 gases not likely to combine with the vapours of the arc, or both 

 these methods can be combined. 



Any experimental means we can employ for studying absorp- 

 tion, should show, when the arc is thus shielded, if my views be 

 correct, that the more we can prolong the existence of these 

 various molecular groupings either by using the arc in an in- 

 tensely heated chamber or by surrounding it with gases or 

 vapours which prevent chemical change, the more apt will the 

 molecules, judged to be finer by other considerations, be to give 

 indications of absorption, and the better will the intermediate 

 stages be observed. The absorption that will then be observed 

 will gradually include that of lines existing generally at the 

 higher temperatures. 



