6 Professor Draper on Tithonized Chlorine. 



There is a great advantage which experiments conducted 

 in this way possess over those which depend for their indica- 

 tion on the stains impressed on Daguerreotype plates or sen- 

 sitive 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 definite chemical effect 

 and the rise of a liquid in a graduated tube; and from this 

 we gain juster views of the true constitution of the spectrum. 

 On studying the numbers in the foregoing table, or better 

 still, if we project them, it will appear what an enormous dif- 

 ference there is in the chemical force of the different rays. 

 In the experiment from which I have deduced this table, it 

 appears that the force of the indigo ray exceeds that of the 

 orange in a greater ratio than 66 to 1 ; and from the circum- 

 stances under which the experiment is made, this difference 

 must be greatly underrated. There is always diffused light 

 in the room coming from the intromitted beam, and this ac- 

 celerates the rise in the less refrangible tubes ; then again, it 

 is impossible that the tube which gives the greatest elevation 

 shall coincide mathematically with the maximum point and 

 express the maximum effect. 



From some estimates I have made, I am led to believe that 

 in point of chemical force, for this mixture of chlorine and 

 hydrogen, the indigo ray exceeds the red in a higher ratio 

 than 500 to 1, 



VI. The action is positive from end to end of the spectrum, 

 M. Becquerel found, that for a Daguerreotype plate the 

 red, the orange, and the yellow rays possess the quality of 

 continuing the action begun by the more refrangible colours ; 

 he therefore names these ^^ rayons continuateurs." For the 

 same compound I found that those rays, acting horizontally 

 with the diffused daylight, exerted a negative agency. It is 

 therefore desirable to understand whether, with respect to the 

 gases now under consideration, the lesser refrangible rays 

 exert anything in the way of an action of depression or hin- 

 drance to union. By direct experiment I found that this was 

 not the case, the action being positive from end to end of the 

 spectrum. This can be shown by removing the tubes, after 

 they have been in the spectrum for an hour or two, into the 

 gleams of daylight. One by one they exhibit after a time a 

 rise, the order being the green first, then the yellow and the 

 orange, and at last the red. And if at the same time a tube 

 which has been kept in the dark be exposed along with them, 

 they will all rise before it, showing that tithonization had set 

 in and been going on in them all ; that it had been more active 



