THIRD  LECTURE.
 

The Atomic Theory of Matter.

The problem with which we shall be occupied in the present
lecture is that of a closer investigation of the atomic theory of
matter. It is, however, not my intention to introduce this
theory with nothing further, and to set it up as something apart
and disconnected with other physical theories, but I intend above
all to bring out the peculiar significance of the atomic theory as
related to the present general system of theoretical physics; for
in this way only will it be possible to regard the whole system
as one containing within itself the essential compact unity, and
thereby to realize the principal object of these lectures.

Consequently it is self evident that we must rely on that sort
of treatment which we have recognized in last week's lecture as
fundamental. That is, the division of all physical processes into
reversible and irreversible processes. Furthermore, we shall be
convinced that the accomplishment of this division is only pos¬
sible through the atomic theory of matter, or, in other words,
that irreversibility leads of necessity to atomistics.

I have already referred at the close of the first lecture to the
fact that in pure thermodynamics, which knows nothing of an
atomic structure and which regards all substances as absolutely
continuous, the difference between reversible and irreversible
processes can only be defined in one way, which a priori carries
a provisional character and does not withstand penetrating anal¬
ysis. This appears immediately evident when, one reflects that
the purely thermodynamic definition of irreversibility which
proceeds from the impossibility of the realization of certain
changes in nature, as, e. g., the transformation of heat into
work without compensation, has at the outset assumed a defi¬
nite limit to man's mental capacity, while, however, such a

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