It is pure in the diamond, nearly pure in coal, and is the principal
constituent of all woody fiber--also of oils, fat, starch, sugar,
etc. Nearly all the visible organic world is composed of carbon. It
appears to be very plentiful, but of our atmosphere it composes only
about four-ten-thousandths, while oxygen, with which it unites to
form carbonic acid gas for vegetation to feed on, composes one-fifth
and nitrogen four-fifths. Really, we have little trouble in securing
carbonaceous foods. The only difficulty is to get them in a digestible
form. Only what is soluble can be digested and assimilated by the
animal organism. Hence, great care must be taken to get food in a
proper condition for animal nutrition.