184 TKOPICAL NATURE, AND OTEER ESSAYS. 



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, and Mr. 

 Sorby has detected no less than seven distinct colouring 

 matters in birds' eggs, several of which are chemically 

 related to those of blood and bile. The same colours are 

 often produced by quite different substances in different 

 groups, as shown by the red of the wing on the burnet- 

 moth changing to yellow with muriatic acid, while the red 

 of the red- admiral-butterfly undergoes no such change. 



These pigmental colours have a different character in 

 animals according to their position in the integument. 

 Following Dr. Hagen's classification, epidermal colours 

 are those which exist in the external chitinised skin of 

 insects, in the hairs of mammals, and, partially, in the 

 feathers of birds. They are often very deep and rich, 

 and do not fade after death. The hypodermal colours 

 are those which are situated in the inferior soft layer of 

 the skin. These are often of lighter and more vivid 

 tints, and usually fade after death. Many of the reds 

 and yellows of butterflies and birds belong to this class, 

 as well as the intensely vivid hues of the naked skin 

 about the heads of many birds. These colours some- 

 times exude through the pores, forming an evanescent 

 bloom on the surface. 



Interference colours are less frequent in the organic 



