v COLOURS OF ANIMALS 857 



produced either by the absorption or by the interference of 

 some of the rays which form white light. Pigmental or 

 absorption-colours are the most frequent, comprising all the 

 opaque tints of flowers and insects, and all the colours of dyes 

 and pigments. They are caused by rays of certain wave- 

 lengths being absorbed, while the remaining rays are reflected 

 and give rise to the sensation of colour. When all the colour- 

 producing rays are reflected in due proportion, the colour of 

 the object is white ; when all are absorbed the colour is black. 

 If blue rays only are absorbed the resulting colour is orange- 

 red ; and generally, whatever colour an object appears to us, 

 it is because the complementary colours are absorbed by it. 

 The reason why rays of only certain refrangibilities are re- 

 flected, and the rest of the incident light absorbed by each 

 substance, is supposed to depend upon the molecular structure 

 of the body. Chemical action almost always implies change 

 of molecular structure; hence chemical action is the most 

 potent cause of change of colour. Sometimes simple solution 

 in water effects a marvellous change, as in the case of the 

 well-known aniline dyes; the magenta and violet dyes 

 exhibiting, when in the solid form, various shades of golden 

 or bronzy metallic green. 



Heat alone often produces change of colour without effect- 

 ing any chemical change. Mr. Ackroyd has investigated this 

 subject, 1 and has shown that a large number of bodies are 

 changed by heat, returning to their normal colour when 

 cooled, and that this change is almost always in the direction 

 of the less refrangible rays or longer wave-lengths; and he 

 connects the change with the molecular expansion caused by 

 heat. As examples may be mentioned mercuric oxide, which 

 is orange yellow, but which changes to orange, red, and brown 

 when heated ; chromic oxide, which is green, and changes to 

 yellow ; cinnabar, which is scarlet, and changes to puce ; and 

 metaborate of copper, which is blue, and changes to green 

 and greenish yellow. 



How Animal Colours are Produced 



The colouring matters of animals are very varied. Copper 

 has been found in the red pigment of the wing of the turaco, 

 1 " Metachromatism, or Colour-Change," Chemical News, August 1876. 



