y as it did to
Hero. The same is true of the argument drawn from the compressibility of
gases. Hero has evidently made a careful study of this subject. He
knows that an inverted tube full of air may be immersed in water without
becoming wet on the inside, proving that air is a physical substance;
but he knows also that this same air may be caused to expand to a much
greater bulk by the application of heat, or may, on the other hand,
be condensed by pressure, in which case, as he is well aware, the air
exerts force in the attempt to regain its normal bulk. But, he argues,
surely we are not to believe that the particles of air expand to
fill all the space when the bulk of air as a whole expands under the
influence of heat; nor can we conceive that the particles of normal air
are in actual contact, else we should not be able to compress the air.
Hence his conclusion, which, as we have seen, he makes general in its
application to all matter, that there are spaces, or, as he calls them,
vacua, between the particles that go to make up all substances, whether
liquid, solid, or gaseous.

Here, clearly enough, was the idea of the "atomic" nature of matter
accepted as a fundamental notion. The argumentative attitude assumed by
Hero shows that the doctrine could not be expected to go unchallenged.
But, on the other hand, there is nothing in his phrasing to suggest an
intention to claim originality for any phase of the doctrine. We may
infer that in the three hundred years that had elapsed since the time
of Anaxagoras, that philosopher's idea of the molecular nature of matter
had gained fairly wide currency. As to the expansive power of gas,
which Hero describes at some length without giving us a clew to his
authorities, we may assume that Ctesibius was an original worker, yet
the general facts involved were doubtless much older than his day. Hero,
for example, tells us of the cupping-glass used by physicians, which
he says is made into a vacuum by burning up the air in it; but this
apparatus had probably been long in use, and Hero mentions it not in
order to describe the ordinary cupping-glass which is referred to, but
a modification of it. He refers to the old form as if it were something
familiar to all.

Again, we know that Empedocles studied the pressure of the air in the
fifth century B.C., and discovered that it would support a column of
water in a closed tube, so this phase of the subject is not new.
But there is no hint