SECT. ii. ELECTIVE POWER OF MATTER. 35 



musical instrument resounds in answer to the note im- 

 pressed upon it. The whole is referable to molecular or 

 atomic motion, for in absorption the vibrations of the 

 ether are communicated to the atoms, and in radiation, 

 the vibrations are returned again to the ether. This 

 principle is known as the law of exchange. 5 



Matter has a decomposing and an elective power with 

 regard to both radiant light and heat; most coloured 

 bodies, such as flowers, green leaves, dyed cloth, &c., 

 though seen by reflection, owe their colour to absorption. 

 The light by which they are seen is reflected, but it is 

 not in reflection that the selection of the rays is made 

 which causes the objects to appear coloured. When 

 light falls upon red cloth, a small portion is reflected at 

 the outer surfaces of the fibres, and this portion, if it 

 could be observed alone, would be found to be colour- 

 less. The greater portion of the light penetrates into 

 the fibres, when it immediately begins to suffer ab- 

 sorption on the part of the colouring matter. On ar- 

 riving at the second surface of the fibre, a portion is re- 

 flected and a portion passes on, to be afterwards reflected 

 from, or absorbed by, fibres lying more deeply. At each 

 reflection the various kinds of light are reflected in as 

 nearly as possible the same proportion, but in passing 

 across the fibres while going and returning they suffer 

 very unequal absorption on the part of the colouring 

 matter ; so that in the aggregation of the light perceived 

 the different components of white light are present in 

 proportions widely different from those they bear to 

 each other in white light itself, and the result is a vivid 

 colouring. 



In certain substances however, as gold and copper, the 

 different components of white light are reflected with 

 different degrees of intensity, and the light becomes 



5 The Law of Exchange was independently proved by Messrs. Tyndall, 

 Kirchhoff, Angstrom, and Balfour Stewart. 



D 2 



