chemistry, but it is necessary
to premise a few observations regarding the organic elements, and their
more important compounds.

_Carbon._--When a piece of wood is heated in a close vessel, it is
charred, and converted into charcoal. This charcoal is the most familiar
form of carbon, but it is not absolutely pure, as it necessarily
contains the ash of the wood from which it was made. In its purest form
it occurs in the diamond, which is believed to be produced by the
decomposition of vegetable matters, and it is there crystallized and
remarkably transparent; but when produced by artificial processes,
carbon is always black, more or less porous, and soils the fingers. It
is insoluble in water, burns readily, and is converted into carbonic
acid. Carbon is the largest constituent of plants, and forms, in round
numbers, about 50 per cent of their weight when dry.

_Carbonic Acid._--This, the most important compound of carbon and
oxygen, is best obtained by pouring a strong acid upon chalk or
limestone, when it escapes with effervescence. It is a colourless gas,
extinguishing flame, incapable of supporting respiration, much heavier
than atmospheric air, and slightly soluble in water, which takes up its
own volume of the gas. It is produced abundantly when vegetable matters
are burnt, as also during respiration, fermentation, and many other
processes. It is likewise formed daring the decay of animal and
vegetable matters, and is consequently evolved from dung and compost
heaps.

_Hydrogen_ occurs in nature only in combination. Its principal compound
is water, from which it is separated by the simultaneous action of an
acid, such as sulphuric acid and a metal, in the form of a transparent
gas, lighter than any other substance. It is very combustible, burns
with a pale blue flame, and is converted into water. It is found in all
plants, although in comparatively small quantity, for, when dry, they
rarely contain more than four or five per cent. Its most important
compound is water, of which it forms one-ninth, the other eight-ninths
consisting of oxygen.

_Nitrogen_ exists abundantly in the atmosphere, of which it forms nearly
four-fifths, or, more exactly, 79 per cent. It is there mixed, but not
combined with oxygen; and when the latter gas is removed, by introducing
into a bottle of air some substance for which the former has an
affinity, the nitrogen is left in a state of purity. It is a transparent
gas, which is incomb