after
treating of Methodical Induction, or the means of determining that a
relation of events is of the nature of cause and effect, because the
relation can be shown to have the marks of causation, or some of them.

Sec. 5. Observations and Experiments are the _material_ grounds of
Induction. An experiment is an observation made under prepared, and
therefore known, conditions; and, when obtainable, it is much to be
preferred. Simple observation shows that the burning of the fire
depends, for one thing, on the supply of air; but it cannot show us that
it depends on oxygen. To prove this we must make experiments as by
obtaining pure oxygen and pure nitrogen (which, mixed in the proportion
of one to four, form the air) in separate vessels, and then plunging a
burning taper into the oxygen--when it will blaze fiercely; and again
plunging it into the nitrogen--when it will be extinguished. This shows
that the greater part of the air does nothing to keep the fire alight,
except by diminishing its intensity and so making it last longer.
Experiments are more perfect the more carefully they are prepared, and
the more completely the conditions are known under which the given
phenomenon is to be observed. Therefore, they become possible only when
some knowledge has already been gained by observation; for else the
preparation which they require could not be made.

Observation, then, was the first material ground of Induction, and in
some sciences it remains the chief ground. The heavenly bodies, the
winds and tides, the strata of the earth, and the movements of history,
are beyond our power to experiment with. Experiments upon the living
body or mind are indeed resorted to when practicable, even in the case
of man, as now in all departments of Psychology; but, if of a grave
nature, they are usually thought unjustifiable. And in political affairs
experiments are hindered by the reflection, that those whose interests
are affected must bear the consequences and may resent them. Hence, it
is in physical and chemical inquiries and in the physiology of plants
and animals (under certain conditions) that direct experiment is most
constantly practised.

Where direct experiment is possible, however, it has many advantages
over unaided observation. If one experiment does not enable us to
observe the phenomenon satisfactorily, we may try again and again;
whereas the mere observer, who wishes to study the bright spots on Mars,
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