232 DOC.
20
THEORETICAL ATOMISM
Doc.
20
Theoretical
Atomism
by
A.
Einstein
[In
Die Kultur der
Gegenwart.
Ihre
Entwicklung
und Ihre
Ziele,
ed.
Paul
Hinneberg,
Part
3, sec. 3,
vol.
1,
Physik,
ed.
Emil
Warburg, Leipzig:
Teubner,
1915,
pp.
251-263]
The development of
the
modem theory of heat
in
general,
as
well as the development
of the molecular-kinetic theory of heat, was initiated primarily
by
the establishment
of
the
energy principle. Let us first dwell briefly on the latter in order
to
consider the
relationship of this principle
to
physical experience. We will carry out this
consideration
using
a
very simple comparison
so as
to
let neither the
multifaceted
nature
of the
topic
nor
habits
or
convictions
get
in
the
way.
I
have before
me
two
vessels
G1
and
G2,
which
are
open
at
the
top
and
partly
filled with
water,
and which communicate with each other via
a
flexible hose
through
which the
water
can
flow from the lowest
layer
of
one
of the vessels into that of the
other;
this will
always
continue until the levels
in
the
two
vessels
(taken absolutely)
have the
same
height.
Let the vessel
G1
have
transparent
walls,
so
that its
water
level
can
be observed from the
outside,
and let it
be
firmly
fixed
at
a
certain
height.
Let
the walls of the second vessel be
opaque,
and let
it be
absolutely impossible
to
perceive
the
water
level in
this
vessel
directly;
but this vessel shall be
in
the vertical
direction,
and there shall
be
some
way
of
determining
the
weight
of the
second vessel
together
with
its contents at
every height.
After each
of
G2,
once
equilibrium
has been
established,
an
observer shall determine the
water
level
h
in
G1
with
respect
to
the walls of this
vessel,
as
well
as
the
corresponding weight
g
of
G2,
restricting
himself
to
those
settings
of
G2
in
which there is
water
in
G1
as
well
as
in
G2.
If vessel
G1
is
cylindrical,
this observer will find
a
simple relationship,
namely
that-given
an appropriate
choice of the numerical factor
a-the
quantity
ah
+ g
is
independent
of the
height
at which
G2
has been
set.
If the observer knows the
laws of
hydrostatics
and knows that
the vessels contain
a
liquid,
he will find these
experiments uninteresting.
But let
our
observer know
nothing
contents
of
the
vessels;
for him the result
concerning
the
physical
system
he
has been
studying
represents
a
scientific
discovery.
He will
say:
"The
water
level
h
of
G1
and the
weight
g
of
G2
are
equivalent quantities,
since
a
specific change
of the
water
level
h
of
G1
always
results
in
a
wholly
determinate,
opposite change
of
g;
the
quantity
ah
+ g
measures an
invariant
quantity
characteristic of the
system."
Repeated findings
of
an
analogous
kind
are
what led
physicists
to
the
principle
of the conservation of
energy.
It
was
first discovered
in
the domain of
pure
A
Comparison
Illustrating the
Relation of the
Energy
Principle
to
Experiment.
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