1868.] 



Chemical Reactions produced by Light. 



95 



was also more feebly acted on through the divergence of the rays. On re- 

 versing the tube, a second and similar cone was precipitated. 



Physical considerations. 



I sought to determine the particular portion of the white beam which 

 produced the foregoing effects. When, previous to entering the experi- 

 mental tube, the beam was caused to pass through a red glass, the effect 

 was greatly weakened, but not extinguished. This was also the case with 

 various samples of yellow glass. A blue glass being introduced, before the 

 removal of the yellow or the red, on taking the latter away augmented 

 precipitation occurred along the track of the blue beam. Hence, in this 

 case, the more refrangible rays are the most chemically active. 



The colour of the liquid nitrite of amyl indicates that this must be the 

 case ; it is a feeble but distinct yellow : in other words, the yellow portion 

 of the beam is most freely transmitted. It is not, however, the transmitted 

 portion of any beam which produces chemical action, but the absorbed por- 

 tion. Blue, as the complementary colour to yellow, is here absorbed, and 

 hence the more energetic action of the blue rays. This reasoning, how- 

 ever, assumes that the same rays are absorbed by the liquid and its va- 

 pour. 



A solution of the yellow chromate of potash, the colour of which may be 

 made almost, if not altogether, identical with that of the liquid nitrite of 

 amyl, was found far more effective in stopping the chemical rays than 

 either the red or the yellow glass. But of all substances the nitrite itself 

 is most potent in arresting the rays which act upon its vapour. A layer 

 one- eighth of an inch in thickness, which scarcely perceptibly affected the 

 luminous intensity, sufficed to absorb the entire chemical energy of the con- 

 centrated beam of the electric light. 



The close relation subsisting between a liquid and its vapour, as regards 

 their action upon radiant heat, has been already amply demonstrated *. As 

 regards the nitrite of amyl, this relation is more specific than in the cases 

 hitherto adduced ; for here the special constituent of the beam which pro- 

 vokes the decomposition of the vapour is shown to be arrested by the 

 liquid. 



A question of extreme importance in molecular physics here arises : — 

 What is the real mechanism of this absorption, and where is its seat f ? 



I figure, as others do, a molecule as a group of atoms, held to- 

 gether by their mutual forces, but still capable of motion among them- 

 selves. The vapour of the nitrite of amyl is to be regarded as an assemblage 

 of such molecules. The question now before us is this : — In the act of ab- 

 sorption, is it the molecules that are effective, or is it their constituent 



* Phil. Trans. 1864. 



t My attention was very forcibly directed to this subject some years ago by a con- 

 versation with my excellent friend Professor Clausius. 



