th the latter doctrine
(Deschanel: Sec. 813).

Perhaps experiments of this decisive character are commonest in
Chemistry: chemical tests, says Herschel, "are almost universally
crucial experiments." The following is abridged from Playfair (_Encycl.
Met., Diss._ III.): The Chemists of the eighteenth century observed that
metals were rendered heavier by calcination; and there were two ways of
accounting for this: either something had been added in the process,
though what, they could not imagine; or, something had been driven off
that was in its nature light, namely, phlogiston. To decide between
these hypotheses, Lavoisier hermetically sealed some tin in a glass
retort, and weighed the whole. He then heated it; and, when the tin was
calcined, weighed the whole again, and found it the same as before. No
substance, therefore, either light or heavy, had escaped. Further, when
the retort was cooled and opened, the air rushed in, showing that some
of the air formerly within had disappeared or lost its elasticity. On
weighing the whole again, its weight was now found to have increased by
ten grains; so that ten grains of air had entered when it was opened.
The calcined tin was then weighed separately, and proved to be exactly
ten grains heavier than when it was placed in the retort; showing that
the ten grains of air that had disappeared had combined with the metal
during calcination. This experiment, then, decided against phlogiston,
and led to an analysis of common air confirming Priestley's discovery of
oxygen.

(4) _An hypothesis must agree with the rest of the laws of Nature; and,
if not itself of the highest generality, must be derivable from primary
laws_ (chap. xix. Sec. 1). Gravitation and the diffusion of heat, light and
sound from a centre, all follow the 'law of the inverse square,' and
agree with the relation of the radius of a sphere to its surface. Any
one who should think that he had discovered a new central force would
naturally begin to investigate it on the hypothesis that it conformed to
the same law as gravitation or light. A Chemist again, who should
believe himself to have discovered a new element, would expect it to
fill one of the vacant places in the Periodic Table. Conformity, in such
cases, is strong confirmation, and disagreement is an occasion of
misgivings.

A narrower hypothesis, as 'that the toad's ugliness is protective',
would be supported by the general theory of protective colouring and