ubstance of the plenum. A rough analogy to the latter case would be
afforded by granules of ice diffused through water; to the former,
such granules diffused through absolutely empty space.

[Sidenote: Reassertion by Dalton of atomic theory.]

In the latter part of the eighteenth century, the chemists had arrived
at several very important generalisations respecting those properties
of matter with which they were especially concerned. However plainly
ponderable matter seemed to be originated and destroyed in their
operations, they proved that, as mass or body, it remained
indestructible and ingenerable; and that, so far, it varied only in
its perceptibility by our senses. The course of investigation further
proved that a certain number of the chemically separable kinds of
matter were unalterable by any known means (except in so far as they
might be made to change their state from solid to fluid, or _vice
versa_), unless they were brought into contact with other kinds of
matter, and that the properties of these several kinds of matter were
always the same, whatever their origin. All other bodies were found to
consist of two or more of these, which thus took the place of the four
'elements' of the ancient philosophers. Further, it was proved that,
in forming chemical compounds, bodies always unite in a definite
proportion by weight, or in simple multiples of that proportion, and
that, if any one body were taken as a standard, every other could have
a number assigned to it as its proportional combining weight. It was
on this foundation of fact that Dalton based his re-establishment of
the old atomic hypothesis on a new empirical foundation. It is
obvious, that if elementary matter consists of indestructible and
indivisible particles, each of which constantly preserves the same
weight relatively to all the others, compounds formed by the
aggregation of two, three, four, or more such particles must exemplify
the rule of combination in definite proportions deduced from
observation.

In the meanwhile, the gradual reception of the undulatory theory of
light necessitated the assumption of the existence of an 'ether'
filling all space. But whether this ether was to be regarded as a
strictly material and continuous substance was an undecided point, and
hence the revived atomism, escaped strangling in its birth. For it is
clear, that if the ether is admitted to be a continuous material
substance, Democritic atomism is at an end a