356 Prof. Draper on the Allotropism of Chlorine 
If, therefore, this phenomenon is 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 dispersing the ray by a flint 
glass prism, received the resulting spectrum on a daguerreotype 
plate. Simultaneously, by the side of it, I received the spectrum 
of aray 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 absorp- 
tive 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 ter- 
mination 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 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 affected by all those waves whose 
lengths are between 0:00001587 and 0-00001287 of a Paris 
inch ; and inasmuch as it absorbs photic rays included between 
the same limits, it is to this absorption that its yellow color is due. 
In the Philosophical Magazine the same result was establish- 
ed by me in another way. I found that a ray which had passed 
through a given thickness of a mixture of equal volumes of 
chlorine and hydrogen, lost by absorption just half as much of 
its original intensity as when it passed through the same thick- 
ness of pure chlorine gas ; a result which obviously leads to the 
conclusion that when chlorine and hydrogen unite, under the 
influence of the sun, they discharge functions which are differ- 
ent—the chlorine an active and the hydrogen a passive function. 
The primary action or disturbance takes place upon the chlo- 
rine, and a disposition is communicated to it enabling it to unite 
readily with the hydrogen. 
