276 MODIFIED CHLORINE. [MEMOIR XIX. 



may be caused to traverse. The active rays have been 

 absorbed; they disappear from .the beam and are lost in 

 producing their first effect. 



A beam of light loses its energy in producing a chem- 

 ical effect ; the beam, as well as the medium on which it 

 acts, becomes changed. I have a series of results which 

 proves that this takes place for a great variety of com- 

 pound bodies. 



V. It is the Indigo Hay which is Absorbed. 



As has been said, it is a ray corresponding in refrangi- 

 bility to the indigo which produces these results. 



In a small porcelain trough I inverted, side by side, 

 ten tubes, each of which was three inches long and one 

 third of an inch in diameter, the trough being filled with 

 salt water. I passed into each tube a certain quantity 

 of unmodified chlorine and hydrogen. A beam of the 

 sun, being directed by a heliostat into a dark room, was 

 dispersed horizontally by a flint-glass prism, and the 

 trough with its tubes so placed as to offer an exposure 

 to the different colored rays. The aperture admitting 

 the beam was about half an inch in diameter. For a 

 while no movement was observed in any of the tubes ; 

 but as soon as the preliminary absorption previously de- 

 scribed was over, the level of the liquid began to rise. 

 In the red and in the orange no movement could be 

 perceived, in the violet only after a time ; but first of all 

 the tube that was immersed in the indigo light was in 

 action, and exhibited finally a very rapid rise ; this was 

 soon followed by the tube that was in the space where 

 the indigo and violet joined, then by that in the violet 

 and that in the blue ; the tube in the green was next in 

 order. The following table gives the numerical results 

 obtained by observing the time which elapsed before 

 movement took place in each tube : 



