MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 427 



sity of the lamp had changed. It gave 12 seconds as 

 before. 



From these observations it may be concluded that the 

 addition of hydrogen to chlorine does not increase its 

 absorptive power. Moreover, it is obvious that the ac- 

 tion of the radiation is expended primarily on the chlo- 

 rine, giving it a disposition to unite with the hydrogen, 

 and that the functions discharged by the chlorine and 

 by the hydrogen respectively are altogether different. 

 The ray itself also undergoes a change ; it suffers absorp- 

 tion and loss of a part of its vis viva. 



As to the ray which is thus absorbed. In 1835 I 

 found that a radiation which had passed through a solu- 

 tion of potassium bichromate failed to accomplish the 

 union of chlorine and hydrogen ; but one which had 

 passed through ammonia sulphate of copper could do it 

 energetically. This indicates that the effective rays are 

 among the more refrangible. On exposing these gases 

 in the spectrum, the maximum action takes place in the 

 indigo rays (Philosophical Magazine, Dec., 1843), Memoir 

 XVIII. 



Recently (1871) some suggestions have been made by 

 M. Budde respecting the action of light upon chlorine. 

 Admitting the correctness of the theorem that the mole- 

 cules of most elementary gases consist of two atoms, he 

 conceives that the effect of light on chlorine is to tend 

 to divide, or actually to divide, its molecule into isolated 

 atoms. These atoms, if the gas be kept in the dark, 

 may reunite into molecules. 



The chlorine molecule cannot unite with hydrogen; 

 the chlorine atom can ; hence insolation brings on com- 

 bination. But if the chlorine be unmixed, there will, as 

 a consequence of insolation, be a certain proportion of 

 uncombined atoms; and from this, together with Avo- 

 gadro's theorem, is drawn the conclusion that this gas 



