white glass. The stirring up of water from two pails would not
really mix them but only entangle filaments from the pails.

"To come to the case of energy. All our ideas concerning energy
seem to require that it is capable of gradual increase. Thus the
energy due to velocity can increase continuously if velocity can.
Since the energy is as the square of the velocity, if the velocity
can only increase discontinuously by equal increments, the energy
of the body will increase by unequal increments in such a way as
to make the exchange of energy between bodies a very awkward
matter to adjust.

"But apart from the question of the increase of energy by
discontinuous increments, the question of relative and absolute
motion makes it very hard to give a particular position to energy,
since the 'energy' we speak of in any case is not one quantity but
may be interpreted in a great many ways. Take the important case
of two equal elastic balls. One, moving, strikes the other at
rest, we say, and gives it nearly all its energy. But we have no
right to call one ball at rest and we can not say (as anything
absolute) which of the balls has lost and which has gained energy.
If there is such a thing as absolute energy of motion it is
something entirely unknowable to us. Take the solar system,
supposed isolated. We may take as our origin of coordinates the
center of gravity of the system. Or we may take an origin with
respect to which the center of gravity of the solar system has any
(constant) velocity. The kinetic energy of the earth, for example,
may have any value whatever, and the principle of the conservation
of energy will hold in any case for the whole solar system. But
the shifting of energy from one planet to another will take place