THE INDIGO RAY IS ABSORBED. 



201 



V. It is the Indigo Ray which is absorbed. 



915. As has been said, it is a ray which corresponds in refrangibility to the indigo 

 which produces these results. 



916. 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 untithonized 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 coloured rays. The aperture which admitted 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 described, was 

 over, and the tithonization completed, 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 re- 

 sults obtained by observing the time which elapsed before movement took place in 

 each tube : 



TABLE I. 



917. Many years ago, M. BERARD made experiments on the explosion of chlorine and 

 hydrogen, and concluded, from his results, that it was brought about by the violet ray. 

 This was at a time when the methods of making these experiments were less exactly 

 known. It is a very easy matter to prove that, in reality, the indigo is the active ray, 

 and that, from a maximum point which is in the indigo, but towards the blue, the effect 

 gradually diminishes to each end of the spectrum. 



918. The following table gives the calculated approximate intensity of the chemical 

 force for each ray, deduced from the foregoing experiment : 



TABLE II. 



919. There is a great advantage which experiments conducted in this way possess 

 over those which depend for their indication on the stains impressed on Daguerreotype 

 plates or sensitive papers. In those cases we obtain merely a comparative contrast for 

 different regions of the spectrum ; in this we have absolute measures determined by a 



* Even after the longest exposure I had the means of giving it, no movement took place in the tube which was in the 

 extreme red, and I am doubtful about that in the red and orange. 



C c 



