as connected mth the Theory of Substitutions. 335 



simpler conditions, and study the relations of each of these 

 substances to each other and to the solar rays successively. 



When a mixture of oxygen and hydrogen gases, in the pro- 

 portion to form water, is exposed to the most brilliant radia- 

 tion converged upon it by convex lenses, union does not ensue; 

 the reason being, as I have formerly shown, that those gases 

 are perfectly transparent to the rays, and do not possess either 

 real or ideal coloration. 



For the same cause, water exposed alone for any length of 

 time to the sun, or to the influence of a large convex lens, 

 does not decompose. It is transparent and cannot absorb 

 any of the rays. 



But, as is well known, a mixture of chlorine and hydrogen 

 unites under the same circumstances with an explosion. I 

 have formerly proved that this depends on the absorption of 

 the indigo rays, for in the indigo space the action goes on 

 with the greatest rapidity. 



If, therefore, this phasnomenon is due to absorption taking 

 place by the mixture, it is easy to determine the function dis- 

 charged by each of its ingredients. 



I transmitted a ray of light through hydrogen gas contained 

 in a tube seven inches long, the ends of which were terminated 

 by pieces of flat glass, and then dispersing the ray by a flint- 

 glass prism, received the resulting spectrum on a Daguerreo- 

 type plate. Simultaneously, by the side of it, I received the 

 spectrum of a ray which had not gone through hydrogen, but 

 through a similar tube filled with atmospheric air. On com- 

 paring the resulting impressions together, I could find no dif- 

 ierence between them. 



1 therefore infer that hydrogen gas does not exert any 

 absorptive action on the solar rays. 



In one of the foregoing tubes I placed dry chlorine gas, the 

 other containing atmospheric air as before ; and receiving the 

 two spectra side by side on the same Daguerreotype plate, I 

 found that a powerful absorption had been exercised by the 

 chlorine. All the tithonic rays between the fixed line H and 

 the violet termination of the spectrum were removed, and no 

 impression corresponding to their place was left upon the 

 plate. On repeating this experiment, so as to determine with 

 precision the rays which had been absorbed, I found that 

 chlorine absorbs all the rays of the spectrum included between 

 the fixed line i and the violet termination, and is probably ef- 

 fected by all those waves whose lengths are between 0"00001587 

 and 000001287 of a Paris inch, and inasmuch as it absorbs 

 photic rays included between the same limits, it is to their 

 absorption that its yellow colour is due. 



