M.MoiaXX.] THE ALLOTKOPISM ( )l < II l.i >UIXE. .,;,;, 



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 hy- 

 drogen 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 of the spectrum the action goes on with the great- 

 est activity. 



If, therefore, this phenomenon be due to absorption 

 taking place by the mixture, it is easy to determine 

 the function discharged 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, dis- 

 persing the ray by a flint-glass prism, received the result- 

 ing spectrum on a daguerreotype 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 comparing the im- 

 pressions together, I could find no difference between 

 them. 



I 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 da- 

 guerreotype plate, I found that a powerful absorption 

 had been exercised by the chlorine. All the chemical 

 rays between the fixed line H and the violet termination 

 of the spectrum were removed, and no impression cor- 

 responding to their place was left upon the plate. On 

 repeating this experiment so as to determine with pre- 

 cision the rays which had been absorbed, I found that 

 chlorine absorbs all the rays of the spectrum included 



