and contain but little hydrogen; the last named variety of coal is
smokeless, and is therefore largely used for drying malt.

Carbon occurs in nature also in a combined state; limestone, chalk, and
marble contain 12 per cent. of this element. It is also present in the
atmosphere in the form of carbonic acid, and the same compound of carbon
is present in well and river waters, both in the free state and combined
with lime and magnesia. All animal and vegetable organisms contain a
large proportion of carbon as an essential constituent; albumen contains
about 53 per cent., alcohol contains 52 per cent., starch 44 per cent.,
cane sugar 42 per cent., and so on. The presence of carbon in the large
class of bodies known to chemists as carbohydrates, of which starch and
sugar are prominent examples, can be easily demonstrated. If a little
strong sulphuric acid be added to some powdered cane sugar in a glass,
the mass will soon begin to darken in color and swell up, and in the
course of a few minutes a mass of black porous carbon will separate,
which can be purified from the acid by repeated washings; the sugar is
composed of carbon, hydrogen, and oxygen, the two last-named elements
being present in the exact proportion necessary to form water; the
sulphuric acid having a strong affinity for water, removes the hydrogen
and oxygen, and the carbon is then left in a free state.

Carbon forms two compounds with oxygen--carbon monoxide, commonly called
carbonic oxide, and carbon dioxide, commonly called carbonic acid; and
the last-named, being of most importance, will be studied first.

_Carbon Dioxide, or Carbonic Acid, Symbol CO_2_.--Carbonic acid occurs,
as we have already stated, in large quantities in combination with lime
and magnesia, forming immense rock formations of limestone, chalk,
marble, dolomite, etc.; it also issues in a gaseous state from