dency" of its Cause.

Sec. 6. Mill contrasted two forms under which causation appears to us: that
is to say, the conditions constituting a cause may be modified, or
'intermixed' in the effect, in two ways, which are typified respectively
by Mechanical and Chemical action. In mechanical causation, which is
found in Astronomy and all branches of Physics, the effects are all
reducible to modes of energy, and are therefore commensurable with their
causes. They are either directly commensurable, as in the cases treated
of in the consideration of the mechanical powers; or, if different forms
of energy enter into cause and effect, such as mechanical energy,
electrical energy, heat, these different forms are severally reducible
to units, between which equivalents have been established. Hence Mill
calls this the "homogeneous intermixture of effects," because the
antecedents and consequents are fundamentally of the same kind.

In chemical causation, on the other hand, cause and effect (at least, as
they present themselves to us) differ in almost every way: in the act of
combination the properties of elements (except weight) disappear, and
are superseded by others in the compound. If, for example, mercury (a
heavy, silvery liquid) be heated in contact with oxygen (a colourless
gas), oxide of mercury is formed (red precipitate, which is a powder).
This compound presents very different phenomena from those of its
elements; and hence Mill called this class of cases "the heteropathic
intermixture of effects." Still, in chemical action, the effect is not
(in Nature) heterogeneous with the cause: for the weight of a compound
is equal to the sum of the weights of the elements that are merged in
it; and an equivalence has been ascertained between the energy of
chemical combination and the heat, light, etc., produced in the act of
combination.

The heteropathic intermixture of effects is also found in organic
processes (which, indeed, are partly chemical): as when a man eats bread
and milk, and by digestion and assimilation converts them into nerve,
muscle and bone. Such phenomena may make us wonder that people should
ever have believed that 'effects resemble their causes,' or that 'like
produces like.' A dim recognition of the equivalence of cause and effect
in respect of matter and motion may have aided the belief; and the
resemblance of offspring to parents may have helped: but it is probably
a residuum of magical rites; in which to w