Professor Draper on Tilhonized Chlorine. 5 



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 awhile no movement was observed in any of 

 the tubes; but as soon as the preliminary absorption, previ- 

 ously 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 results ob- 

 tained by observing the time which elapsed before movement 

 took place in each tube : — 



Table I. 



Many years ago M. Berard made experiments on the ex- 

 plosion of chlorine and hydrogen, and concluded from his re- 

 sults 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. 



The following Table gives the calculated approximate in- 

 tensity of the chemical force for each ray, deduced from the 

 foregoinff experiment: — 



^ " Table. II. 



* Even after the longest exposure I had the means of giving it, no move- 

 ment took place in the tube which was in the extreme red, and I am doubt- 

 ful about that in the red and orange. 



