the
comets. Phlogiston, again, an agent much in favour with the earlier
Chemists, was found, Whewell tells us, when their theories were tested
by exact weighing, to be not merely non-existent but a minus quantity;
that is to say, it required the assumption of its absolute lightness "so
that it diminished the weight of the compounds into which it entered."
These agents, then, the spheres and phlogiston, have been explained
away, and instead of them we have the laws of motion and oxygen.

(2) _Whether the hypothetical agent be perceptible or not, it cannot be
established as a cause, nor can a supposed law of such an agent be
accepted as sufficient to the given inquiry, unless it is adequate to
account for the effects which it is called upon to explain, at least so
far as it pretends to explain them._ The general truth of this is
sufficiently obvious, since to explain the facts is the purpose of an
hypothesis; and we have seen that Newton gave up his hypothesis that the
moon was a falling body, as long as he was unable to show that the
amount of its deflection from a tangent (or fall) in a given time, was
exactly what it should be, if the Moon was controlled by the same force
as falling bodies on the Earth.

It is important to observe the limitations to this canon. In the first
place, it says that, unless adequate to explain the facts in question,
an hypothesis cannot be '_established_'; but, for all that, such an
hypothesis may be a very promising one, not to be hastily rejected,
since it may take a very long time fully to verify an hypothesis. Some
facts may not be obtainable that are necessary to show the connection of
others: as, for example, the hypothesis that all species of animals
have arisen from earlier ones by some process of gradual change, can be
only imperfectly verified by collecting the fossil remains of extinct
species, because immense depths and expanses of fossiliferous strata
have been destroyed. Or, again, the general state of culture may be such
as to prevent men from tracing the consequences of an hypothesis; for
which reason, apparently, the doctrine that the Sun is the centre of our
planetary system remained a discredited hypothesis for 2000 years. This
should instruct us not to regard an hypothesis as necessarily erroneous
or illegitimate merely because we cannot yet see how it works out: but
neither can we in such a case regard it as established, unless we take
somebody's word for it.

Secondl