436 Dr. J. W. Draper on the Distribution of 



thickness, may leave the ray possessed of a portion of its power. 

 There must be a correspondence between the intensity of the 

 incident ray and the thickness of the absorbing medium to 

 produce a maximum eflPect. 



Though the silver iodide is affected by radiations of every re- 

 frangibility, it is decomposed (so that a subiodide results) only 

 by those of which the wave-length is less than 5000; if in 

 presence of metallic silver (as on the Daguerreotype tablet), the 

 iodine disengaged unites with the free silver beneath. The rays 

 of high refrangibility occasion in it chemical decomposition, 

 those of less refrangibihty physical modification. In the lan- 

 guage of the older theories of actino-chemistry, this substance 

 may be said to exert a selective absorption. In this it illustrates 

 the general principle, that it depends on the nature of the pon- 

 derable material presented to radiations, which of them shall be 

 absorbed. 



2nd. 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 covering a test-tube contain- 

 ing an explosive mixture of equal volumes of those gases with 

 a large jar filled with chlorine. This arrangement may be 

 exposed in the open daylight without risk of exploding the mix- 

 ture ; but if the experiment be made with a covering jar con- 

 taining atmospheric air instead of chlorine, the gases immediately 

 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 saturated 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 selec- 

 tive absorption of the mixture. They were then received on a 

 chlorhydr.ogen actinometer. 



Successive experiments were then made (1) with the radia- 

 tions of a lamp after passing through the absorption-vessel, (2) 

 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 ; 2nd, the rays which had 

 passed through that mixture had lost very much of their che- 

 mical force. It was not totally extinct; but the actinometer 

 showed that it had undergone a very great diminution. 



From this it follows that, on its passage through a mixture of 

 chlorine and hydrogen, the radiation had suffered absorption, 



