Mi M..IIS XXIX.] ( IIKMICAL FORCE IN THE SPECTRUM. 425 



tangibility, physical modification. In the language of 

 the older theories of actino-chemistry, 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 those 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 chlorine, 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; *J1, 

 the rays which had passed through that mixture had lost 



