78 ABSORPTIVE ACTION OF CHLORINE AND HYDROGEN. 



To the first of these the present chapter is devoted ; the following chapter to the 

 second. 



291. When a beam of light has fallen on any changeable surface, such as a Daguer- 

 reotype plate, and is reflected by it, that beam will be found to have impressed a change 

 on the sensitive surface, greater or less in amount, according to the period of its action. 

 In effecting this it also suffers a change itself, and if received on a second similar sensi- 

 tive surface, is found to have lost the quality of giving rise to the decomposition again. 

 Two changes have, therefore, occurred, a change in the ponderable body, and a change 

 in the incident beam. The particular experiments in proof of this fact are given in 

 AP., 595. 



292. Again, let us take a second instance. As is shown in AP., CH. XVI., a mix- 

 ture of chlorine and hydrogen in equal volumes undergoes combination by the influ- 

 ence of the rays of a lamp, and a rapid action, amounting to an explosion, by the 

 brighter beams of the sun. As these gases can be obtained in a state of uniform purity, 

 and their combination is attended with mechanical results, this forms a favourable case 

 for a minute investigation. With one such clear case to guide us in our researches, we 

 may fall back on it for illustrations, as new phenomena arise. 



293. A mixture of chlorine and hydrogen, such as has been referred to, was placed 

 in a vessel made of plate glass, having flat and parallel sides; it was 7 inches high, 2 

 broad, and 2-6 deep. It was so arranged on a small porcelain trough, that it could be 

 used as a gas jar. The rays of an argand lamp, properly situated, were made to pass 

 through it ; they therefore went through a depth of the compound gases of 2-6 inches. In 

 fig. 123, A is the lamp, so adjusted as to burn steadily, B the vessel containing the 

 chlorine and hydrogen, C the porcelain trough, in which was placed a saturated solu- 

 tion of common salt, which acts on the chlorine slowly, and therefore allows us to 

 make any necessary experiments without much change happening in the gases under 

 trial. At D was placed a tithonometer (Ap., CH. XVI.), to receive the rays from the 

 lamp, after they had emerged from the chlorine and hydrogen. 



294. Two separate phenomena were now apparent: first, the mixture of chlorine 

 and hydrogen began to unite under the influence of the rays of the lamp ; second, 

 the rays which had passed through the mixture had lost very much of their chemical 

 power. It was not totally extinct, but the tithonometer showed that it had undergone 

 a very great diminution. 



295. We see, therefore, that on its passage through a mixture of chlorine and hy- 

 drogen, the beam has become detithonized. Simultaneously, and in producing this re- 

 sult, we see that the sensitive mixture has become tithonized. The connexion and 

 sequence of the phenomena are apparent. The beam has undergone a change itself 

 in producing a given change in the ponderable matter. But this is the same conclu- 

 sion that was furnished us by the rougher experiment with iodide of silver above quo- 

 ted (Ap., 595). 



296. At this stage of our inquiries, therefore, we have already fallen upon one of the 

 leading features of the doctrine of absorption ; for we perceive that, whenever tithonic 

 rays produce a change on a sensitive mixture, they must necessarily undergo a change 

 themselves, and become partially or perfectly detithonized. 



