luminaries than those of our system, others with a greater number.
All, however, had necessarily a centre; all as systems were necessarily
spherical.

[152]

As regards the atoms he conceived that when they differed in weight
this must be in respect of a difference in their essential size. In
this he was no doubt combating the notion that the atoms say of lead or
gold were in their substance, taking equal quantities, of greater
weight than atoms of water or air. The difference of weight in objects
depended on the proportion which the atoms in them bore to the amount
of empty space which was interlaced with them. On the other hand, a
piece of iron was lighter yet harder than a piece of lead of equal
size, because of the special way in which the atoms in it were linked
together. There were fewer atoms in it, but they were, in consequence
of their structure and arrangement, more tightly strung.

[153]

In all this Democritus was with great resolution working out what we
may call a strictly mechanical theory of the universe. Even the soul
or life-principle in living creatures was simply a structure of the
finest and roundest (and therefore most nimble) atoms, with which he
compared the extremely attenuated dust particles visible in their
never-ending {79} dance in a beam of light passed into a darkened room.
This structure of exceeding tenuity and nimbleness was the source of
the motion characteristic of living creatures, and provided that
elastic counteracting force to the inward-pressing nimble air, whereby
were produced the phenomena of respiration. Every object, in fact,
whether living or not, kept its form and distinctive existence by its
possession in degree of a kind of soul or spirit of resistance in its
structure, adequate to counteract the pressure of external forces upon
its particles.

[155]

Sensation and perception were forms in which these external forces
acted upon the more nimble and lively existences, more particularly on
living creatures. For every body was continually sending forth
emanations or images resembling itself sufficiently in form and
structure to affect perceptive bodies with an apprehension of that form
and structure. These images travelled by a process of successive
transmission, similar to that by which wave-motions are propagated in
water. They were, in other words, not movements of the _particles_ of
the objects, which latter must otherwise in time grow less and fade
a