geneous into the heterogeneous.

Thus much premised, we pass at once to the statement of the law, which
is this:--_Every active force produces more than one change_--_every
cause produces more than one effect_.

Before this law can be duly comprehended, a few examples must be looked
at. When one body is struck against another, that which we usually
regard as the effect, is a change of position or motion in one or both
bodies. But a moment's thought shows us that this is a careless and very
incomplete view of the matter. Besides the visible mechanical result,
sound is produced; or, to speak accurately, a vibration in one or both
bodies, and in the surrounding air: and under some circumstances we call
this the effect. Moreover, the air has not only been made to vibrate,
but has had sundry currents caused in it by the transit of the bodies.
Further, there is a disarrangement of the particles of the two bodies in
the neighbourhood of their point of collision; amounting in some cases
to a visible condensation. Yet more, this condensation is accompanied by
the disengagement of heat. In some cases a spark--that is,
light--results, from the incandescence of a portion struck off; and
sometimes this incandescence is associated with chemical combination.

Thus, by the original mechanical force expended in the collision, at
least five, and often more, different kinds of changes have been
produced. Take, again, the lighting of a candle. Primarily this is a
chemical change consequent on a rise of temperature. The process of
combination having once been set going by extraneous heat, there is a
continued formation of carbonic acid, water, etc.--in itself a result
more complex than the extraneous heat that first caused it. But
accompanying this process of combination there is a production of heat;
there is a production of light; there is an ascending column of hot
gases generated; there are currents established in the surrounding air.
Moreover the decomposition of one force into many forces does not end
here: each of the several changes produced becomes the parent of further
changes. The carbonic acid given off will by and by combine with some
base; or under the influence of sunshine give up its carbon to the leaf
of a plant. The water will modify the hygrometric state of the air
around; or, if the current of hot gases containing it come against a
cold body, will be condensed: altering the temperature, and perhaps the
chemical state,