their correlatives, chemical affinity, motive power, electricity, and
magnetism. The decomposition of plants may, however, be more or less
retarded, and it then takes the form of a destructive distillation,
the constituents reacting upon each other, and forming temporary
combinations, part of which are evolved, and part remain behind. Water
is the great extinguisher of this as of the more rapid oxidation that we
call combustion; and the decomposition of plant-tissue under water is
extremely slow, from the partial exclusion of oxygen. Buried under thick
and nearly impervious masses of clay, where the exclusion of oxygen is
still more nearly complete, the decomposition is so far retarded that
plant-tissue, which is destroyed by combustion almost instantaneously,
and if exposed to "the elements"--moisture with a free access of
oxygen--decays in a year or two, may be but partially consumed when
millions of years have passed. The final result is, however, inevitable,
and always the same, viz., the oxidation and escape of the organic
mutter, and the concentration of the inorganic matter woven into its
composition--in it, but not of it--forming what we call the ash of the
plant.

Since the decomposition of organic matter commences the instant it is
abandoned by the creative and conservative vital force, and proceeds
uninterruptedly, whether slowly or rapidly, to the final result, it is
evident that each moment in the progress of this decomposition presents
us with a phase of structure and composition different from that which
preceded and from that which follows it. Hence the succession of these
phases forms a complete sliding scale, which is graphically shown in
the following diagram, where the organic constituents of plant
tissue--carbon, hydrogen, oxygen, and nitrogen--appear gradually
diminishing to extinction, while the ash remains nearly constant, but
relatively increasing, till it is the sole representative of the fabric.