DECOMPOSITION BY LIGHT. 103 



produces chemical action, but the absorbed portion. 

 Blue, as the complementary colour to yellow, 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 vapour. The 

 assumption is worth testing. 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 liquid nitrite 

 itself is most potent in arresting the rays which act 

 upon its vapour. A layer one-eighth of an inch in thick- 

 ness, which scarcely perceptibly affected the luminous 

 intensity, absorbed the entire chemical energy of the 

 concentrated 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. 1 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 

 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 ? 2 I figure, as others 

 do, a molecule as a group of atoms, held together by 

 their mutual forces, but still capable of motion among 

 themselves. The vapour of the nitrite of amyl is to 



1 Phil. Trans/ 1864 ; Heat, a Mode of Motion,' chap. xii. ; and 

 p. 61 of this volume. 



2 My attention was very forcibly directed to this subject some 

 years ago by a conversation with my excellent friend Professor 



8 



