* 
1877.] The Colors of Animals and Plants. 657 
the magenta and violet dyes exhibiting, when in the solid form, 
various shades of golden or bronzy metallic green. Heat, again, 
often produces change of color, and this without effecting any 
chemical change. Mr. Ackroyd has recently investigated this 
subject,! and has shown that a large number of bodies are 
changed by heat, returning to their normal color 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 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. The coloring 
matters of animals are very varied. Copper has been found in 
the red of the wing of the turaco, and Mr. Sorby has detected no 
less than seven distinct coloring matters in birds’ eggs, several of 
which are chemically related to those of blood and bile. The 
same colors are often produced by quite different substances in 
different groups, as shown by .the red of the wings of the burnet 
moth changing to yellow with muriatic acid, while the red of the 
red-admiral butterfly undergoes no such change. 
These pigmental colors have a different character in animals, 
according to their position in the integument. Following Dr. 
Hagen’s classification, epidermal colors are those which exist in 
the external chitinized skin óf 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 colors 
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 colors 
Sometimes exude through the pores, forming an evanescent bloom 
on the surface. 
Interference colors are less frequent in the organic world. They 
ate caused in two ways: either by reflection from the two sur- 
faces of transparent films, as seen in the soap-bubble and in thin 
Ee films of oil on water ; or by fine striæ, which produce colors either 
a » reflected or transmitted light, as seen in mother-of-pearl and 
_ i finely-ruled metallic surfaces. In both cases color is produced 
; Metachromatism, or Color-Change, Chemical News, ’August, 1876. 
è 42 
= n OL. XI. — No. 11 
