COLOURS OF ANIMALS. 183 



ference 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 reflected, 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 

 effecting any chemical change. Mr. Ackroyd has 

 recently 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. 



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



