imilarly with the
boiling point of liquids. That the transmission of sound depends upon
the continuity of an elastic ponderable medium, is proved by letting a
clock strike in a vacuum (under a glass from which the air has been
withdrawn by an air pump), and standing upon a non-elastic pedestal:
when the clock be seen to strike, but makes only such a faint sound as
may be due to the imperfections of the vacuum and the pedestal.

The experiments by which the chemical analysis or synthesis of various
forms of matter is demonstrated are simple or compound applications of
this method of Difference, together with the quantitative mark of
causation (that cause and effect are equal); since the bodies resulting
from an analysis are equal in weight to the body analysed, and the body
resulting from a synthesis is equal in weight to the bodies synthesised.
That an electric current resolves water into oxygen and hydrogen may be
proved by inserting the poles of a galvanic battery in a vessel of
water; when this one change is followed by another, the rise of bubbles
from each pole and the very gradual decrease of the water. If the
bubbles are caught in receivers placed over them, it can be shown that
the joint weight of the two bodies of gas thus formed is equal to the
weight of the water that has disappeared; and that the gases are
respectively oxygen and hydrogen may then be shown by proving that they
have the properties of those gases according to further experiments by
the method of Difference; as (e.g.) that one of them is oxygen because
it supports combustion, etc.

When water was first decomposed by the electric current, there appeared
not only oxygen and hydrogen, but also an acid and an alkali. These
products were afterwards traced to impurities of the water and of the
operator's hands. Mill observes that in any experiment the effect, or
part of it, may be due, not to the supposed agent, but to the means
employed in introducing it. We should know not only the other conditions
of an experiment, but that the agent or change introduced is nothing
else than what it is supposed to be.

In the more complex sciences the method of Difference is less easily
applicable, because of the greater difficulty of being sure that only
one circumstance at a time has altered; still, it is frequently used.
Thus, if by dividing a certain nerve certain muscles are paralysed, it
is shown that normally that nerve controls those muscles. That the se