v COLOURS OF ANIMALS 357 
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 shoavn 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 examplés 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. 
