s--that is, by a separation of the substance to be
examined into its constituent parts. The chemical examination of
bodies is generally effected by producing a change in the _nature_ or
_state_ of the body under examination. This change is frequently
brought about by the addition of some _other_ substance which forms a
combination with a part of the substance examined, and leaves the
remainder in a detached state.

By what _means_ do Chemists effect a change in the qualities or states
of natural bodies?

It is generally effected by means of _heat_, which has a tendency to
separate the particles of bodies from each other; or by the _mixture_
or _combination_ of some other matter with the matter intended to be
examined. The mixture of two or more compounds often produces a
decomposition by means of chemical _affinity_, a property which
different species of matter have to unite with each other; and which
is sometimes called _elective affinity_. Thus it may be observed,
chemists have not only the power of decomposing natural bodies, but of
producing by combination various other substances, such as are not
found in the kingdom of nature.

What do you mean by _decomposition_?

In chemical language, it means the separation of a compound body into
its simple elements.

Give me an example.

Water may be decomposed, and reduced into oxygen and hydrogen,--both
of them simple substances incapable of further decomposition.

Is not the work of decomposition perpetually going forward?

Yes; and _combustion_ is one of the great agents in this work. By it
animal and vegetable substances are converted into water and carbonic
acid, by the union of their hydrogen and carbon with the oxygen of the
air. These, in time, are again absorbed by vegetables, and again
decomposed to set the oxygen at liberty to produce fresh combustions.

Of what use are the two remaining substances, Hydrogen and Carbon?

These are appropriated by the vegetative organs to their growth and
nourishment, while the oxygen with which the carbon was combined is
abundantly given off to purify the air and render it fit for the
respiration of animals.

Give me an idea of the mode in which Chemists ascertain the _affinity_
of bodies, by relating an experiment.

Dissolve a tea-spoonful of sugar of lead in water, and pour the clear
solution into a decanter or large glass bottle. Then take a small
piece of zinc, and twist round it some brass or copper