and motion, capable of assuming? Experience alone
can lead us to a conclusion on this point. That we may experiment to
advantage, we must select implements which, besides causing a real
cessation of motion, are as little as possible altered by the objects
to be examined. For example, if we rub together two metal plates, we see
motion disappear, and heat, on the other hand, make its appearance, and
there remains to be determined only whether MOTION is the cause of heat.
In order to reach a decision on this point, we must discuss the question
whether, in the numberless cases in which the expenditure of motion is
accompanied by the appearance of heat, the motion has not some other
effect than the production of heat, and the heat some other cause than
the motion.

"A serious attempt to ascertain the effects of ceasing motion has never
been made. Without wishing to exclude a priori the hypothesis which
it may be possible to establish, therefore, we observe only that, as a
rule, this effect cannot be supposed to be an alteration in the state of
aggregation of the moved (that is, rubbing, etc.) bodies. If we assume
that a certain quantity of motion v is expended in the conversion of a
rubbing substance m into n, we must then have m + v - n, and n = m + v;
and when n is reconverted into m, v must appear again in some form or
other.

"By the friction of two metallic plates continued for a very long time,
we can gradually cause the cessation of an immense quantity of movement;
but would it ever occur to us to look for even the smallest trace of the
force which has disappeared in the metallic dust that we could collect,
and to try to regain it thence? We repeat, the motion cannot have been
annihilated; and contrary, or positive and negative, motions cannot be
regarded as = o any more than contrary motions can come out of nothing,
or a weight can raise itself.

"Without the recognition of a causal relation between motion and heat,
it is just as difficult to explain the production of heat as it is
to give any account of the motion that disappears. The heat cannot be
derived from the diminution of the volume of the rubbing substances.
It is well known that two pieces of ice may be melted by rubbing them
together in vacuo; but let any one try to convert ice into water by
pressure, however enormous. The author has found that water undergoes
a rise of temperature when shaken violently. The water so heated (from
twelve to thirteen deg