MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 425 



frangibility, physical modification. In the language of 

 the older theories of actino-chernistry, this substance may 

 be said to exert a selective absorption. In this it illus- 

 trates the general principle, that it depends on the nat- 

 ure of the ponderable material presented to radiations 

 which of them shall be absorbed. 



2d. Of the Union of Chlorine and Hydrogen. 



^An interesting experiment, illustrating the fact that 

 chlorine gas absorbs the radiations which bring about 

 its combination with hydrogen, may be made by cover- 

 ing a test-tube containing an explosive mixture of equal 

 volumes of thofee gases with a large jar filled with chlo- 

 rine. This arrangement may be exposed in the open 

 daylight without risk of exploding the mixture ; but if 

 the experiment be made with a covering jar containing 

 atmospheric air instead of chlorin,e, the gases immediate- 

 ly unite, and commonly with an explosion. 



I placed a mixture of equal volumes of chlorine and 

 hydrogen in a vessel made of plate-glass, the edges of 

 the pieces being cemented together. This vessel was so 

 arranged on a small porcelain trough, containing a satu- 

 rated solution of common salt, that it could be used as a 

 gas-jar. The radiations of a lamp were caused to pass 

 through it, so as to be submitted to the selective absorp- 

 tion of the mixture. They were then received on a chlor- 

 hydrogen actinometer. 



Successive experiments were then made : 1st, with the 

 radiations of a lamp after passing through the absorption 

 vessel ; 2d, with the same radiations after the vessel had 

 been removed. 



Two facts were now apparent: 1st, the mixture of 

 chlorine and hydrogen in the absorption vessel began to 

 unite under the influence of the rays of the lamp ; 2d, 

 the rays which had passed through that mixture had lost 



