160 ABSORPTION OF LIGHT. SECT. XIX. 



always greater than the sum of the reflected and refracted rays. 

 A small quantity is irregularly reflected in all directions by the 

 imperfections of the polish by which we are enabled to see the 

 surface ; but a much greater portion is absorbed by the body. 

 Bodies that reflect all the rays appear white, those that absorb 

 them all seem black ; but most substances, after decomposing the 

 white light which falls upon them, reflect some colours and 

 absorb the rest. A violet reflects the violet rays alone and 

 absorbs the others. Scarlet cloth absorbs almost all the colours 

 except red. Yellow cloth reflects the yellow rays most abun- 

 dantly, and blue cloth those that are blue. Consequently colour 

 is not a property of matter, but arises from the action of matter 

 upon light. In fact, the law of action and reaction obtains in 

 light as in every other department of nature, so that light cannot 

 be reflected, refracted, much less absorbed, by any medium 

 without being reacted upon by it. Thus a white riband reflects 

 all the rays, but, when dyed red, the particles of the silk acquire 

 the property of reflecting the red rays most abundantly and of 

 absorbing the others. Upon this property of unequal absorption 

 the colours of transparent media depend ; for they also receive 

 their colour from their power of stopping or absorbing some of 

 the colours of white light, and transmitting others. As, for 

 example, black and red inks, though equally homogeneous, absorb 

 different kinds of rays ; and, when exposed to the sun, they 

 become heated in different degrees ; while pure water seems to 

 transmit all rays equally, and is not sensibly heated by the passing- 

 light of the sun. The rich dark light transmitted by a smalt-blue 

 finger-glass is not a homogeneous colour like the blue or indigo 

 of the spectrum, but is a mixture of all the colours of white light 

 which the glass has not absorbed. The colours absorbed are such 

 as mixed with the blue tint would form white light. When the 

 spectrum of seven colours is viewed through a thin plate of this 

 glass, they are all visible ; and, when the plate is very thick, 

 every colour is absorbed between the extreme red and the extreme 

 violet, the interval being perfectly black ; but, if the spectrum be 

 viewed through a certain thickness of the glass intermediate 

 between the two, it will be found that the middle of the red 

 space, the whole of the orange, a great part of the green, a con- 

 siderable part of the blue, a little of the indigo, and a very little 

 of the violet, vanish, being absorbed by the blue glass ; and that 



