llO FRAGMENTS OF SCIENCE 



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 ab- 

 sorbed portion. Blue, as the complementary color to yel- 

 low, is here absorbed, and hence the more energetic action 

 of the blue rays. 



This reasoning, however, assumes that the same rays 

 are absorbed by the liquid and its vapor. The assump- 

 tion is worth testing. A solution of the yellow chromate 

 of potash, the color 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 liquid nitrite itself is most potent in arrest- 

 ing the rays which act upon its vapor. A layer one- 

 eighth of an inch in thickness, which scarcely perceptibly 

 affected the luminous intensity, absorbed the entire chem- 

 ical energy of the concentrated beam of the electric light. 



The close relation subsisting between a liquid and its 

 vapor, 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 provokes the decomposition of the vapor, 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 absorp- 

 tion, and where is its seat?' I figure, as others do, a 



* "Phil. Trans.** 1864; "Heat, a Mode of Motion,*' chap. xii. ; and p. 67 

 of this volume. 



' My attention was very forcibly directed to this subject some years ago by 

 a conversation with my excellent friend Professor Clausius. 



