tudes as different forms under which one and the same
object makes its appearance. This capability of assuming various forms
is the second essential property of all causes. Taking both properties
together, we may say, causes an INDESTRUCTIBLE quantitatively, and
quantitatively CONVERTIBLE objects.

"There occur in nature two causes which apparently never pass one into
the other," said Mayer. "The first class consists of such causes as
possess the properties of weight and impenetrability. These are kinds of
matter. The other class is composed of causes which are wanting in the
properties just mentioned--namely, forces, called also imponderables,
from the negative property that has been indicated. Forces are therefore
INDESTRUCTIBLE, CONVERTIBLE, IMPONDERABLE OBJECTS.

"As an example of causes and effects, take matter: explosive gas, H + O,
and water, HO, are related to each other as cause and effect; therefore
H + O = HO. But if H + O becomes HO, heat, cal., makes its appearance
as well as water; this heat must likewise have a cause, x, and we have
therefore H + O + X = HO + cal. It might be asked, however, whether H
+ O is really = HO, and x = cal., and not perhaps H + O = cal., and x =
HO, whence the above equation could equally be deduced; and so in many
other cases. The phlogistic chemists recognized the equation between
cal. and x, or phlogiston as they called it, and in so doing made a
great step in advance; but they involved themselves again in a system of
mistakes by putting x in place of O. In this way they obtained H = HO +
x.

"Chemistry teaches us that matter, as a cause, has matter for its
effect; but we may say with equal justification that to force as a cause
corresponds force as effect. Since c = e, and e = c, it is natural to
call one term of an equation a force, and the other an effect of force,
or phenomenon, and to attach different notions to the expression force
and phenomenon. In brief, then, if the cause is matter, the effect is
matter; if the cause is a force, the effect is also a force.

"The cause that brings about the raising of a weight is a force. The
effect of the raised weight is, therefore, also a force; or, expressed
in a more general form, SEPARATION IN SPACE OF PONDERABLE OBJECTS IS
A FORCE; and since this force causes the fall of bodies, we call it
FALLING FORCE. Falling force and fall, or, still more generally,
falling force and motion, are forces related to each other as cause and