to its source or in which credit is given to any one for a
discovery. This is the more to be regretted because Hero has
discussed at some length the theories involved in the treatment
of his subject. This reticence on the part of Hero, combined with
the fact that such somewhat later writers as Pliny and Vitruvius
do not mention Hero's name, while they frequently mention the
name of his master, Ctesibius, has led modern critics to a
somewhat sceptical attitude regarding the position of Hero as an
actual discoverer.

The man who would coolly appropriate some discoveries of others
under cloak of a mere prefatorial reference was perhaps an
expounder rather than an innovator, and had, it is shrewdly
suspected, not much of his own to offer. Meanwhile, it is
tolerably certain that Ctesibius was the discoverer of the
principle of the siphon, of the forcing-pump, and of a pneumatic
organ. An examination of Hero's book will show that these are
really the chief principles involved in most of the various
interesting mechanisms which he describes. We are constrained,
then, to believe that the inventive genius who was really
responsible for the mechanisms we are about to describe was
Ctesibius, the master. Yet we owe a debt of gratitude to Hero,
the pupil, for having given wider vogue to these discoveries, and
in particular for the discussion of the principles of
hydrostatics and pneumatics contained in the introduction to his
book. This discussion furnishes us almost our only knowledge as
to the progress of Greek philosophers in the field of mechanics
since the time of Archimedes.

The main purpose of Hero in his preliminary thesis has to do with
the nature of matter, and recalls, therefore, the studies of
Anaxagoras and Democritus. Hero, however, approaches his subject
from a purely material or practical stand-point. He is an
explicit champion of what we nowadays call the molecular theory
of matter. "Every body," he tells us, "is composed of minute
particles, between which are empty spaces less than these
particles of the body. It is, therefore, erroneous to say that
there is no vacuum except by the application of force, and that
every space is full either of air or water or some other
substance. But in proportion as any one of these particles
recedes, some other follows it and fills the vacant space;
therefore there is no continuous vacuum, except by the
application of some force [like suction]--that is to say, an
absolute vacuum is never found, except as it is produced
artificially." Hero brings forward some thoroughly convincing
proofs of the thesis he is maintaining. "If there were no void
places between the particles of water," he says, "the rays of
light could not penetrate the water; moreover, another liquid,
such as wine, could not spread itself through the water, as it is
observed to do, were the particles of water absolutely
continuous." The latter illustration is one the validity of which
appeals as forcibly to the physicists of to-day 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 anywhere before this work of Hero
of a clear understanding that the expansive properties of the air
when compressed, or when heated, may be made available as a motor
power. Hero, however, has the clearest notions on the subject and
puts them to the practical test of experiment. Thus he constructs
numerous mechanisms in which the expansive power of air under
pressure is made to do work, and others in which the same end is


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