202 THE ACTION IS POSITIVE THROUGH THE SPECTRUM. 



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 enor- 

 mous difference 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 ex- 

 ceeds that of the orange in a greater ratio than 66 to 1 ; and from the circumstances 

 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 accel- 

 erates 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. 



920. From some estimates I have made, I am led to believe that, in point of chem- 

 cal force, for this mixture of chlorine and hydrogen, the indigo ray exceeds the red 

 in a higer ratio than 500 to 1. 



VI. The Action is positive from End to End of the Spectrum. 



921. 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 refran- 

 gible colours ; he therefore names these " rayons continuateurs." For the same com- 

 pound I found that those rays, acting conjointly 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 hinderance 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 titbonization had set in and been 

 going on in them all ; that it had been more active in the green than in the yellow, in 

 the yellow than in the orange, in the orange than in the red ; and had the exposure to 

 the spectrum been long enough, the liquid in every one of the tubes would have risen. 



VII. The Indigo Ray forms the Muriatic Acid, as well as produces the preliminary 



Tithonization. 



922. It only now remains to inquire, whether the rays which cause the production 

 of the muriatic acid are those which effect the tithonization of the chlorine ; in other 

 words, whether the first stage of the process is brought about by the same agent which 

 carries on the second. The experiment which I have just described shows that 

 tithonization is most actively produced by the indigo ray, and it is easy to show that it 

 is the same ray which carries on the second part of the process ; for if, before placing 

 the tubes in the prismatic spectrum, we tithonize them in the daylight, so that the liquid 

 has just commenced to rise in each, and then expose them to the spectrum, it will be 



