Fe_{3}O_{4} + 4H_{2} → 3Fe + 4H_{2}O.
Many other well-known and important facts, both chemical and physical,
depend upon this law. It explains the circumstance that a vapor-pressure
is not dependent upon the amount of the liquid that is present; it also
explains the constant dissociation pressure of calcium carbonate at a
given temperature, irrespective of the amounts of carbonate and oxide
present; in connection with the ionic theory, it furnishes the reason
for the variable solubility of salts due to the presence of electrolytes
containing ions in common; and it elucidates Henry’s law which states
that the solubilities of gases are proportional to their pressures.
Ostwald, more than any other chemist, has been instrumental in making
general applications of this law, and he made particularly extensive use
of it in connection with analytical chemistry in a book upon this
subject which he published.
_The Phase Rule._—In 1876 Willard Gibbs of Yale published a paper in the
Proceedings of the Connecticut Academy of Science on the “Equilibrium of
Heterogeneous Substances,” and two years later he published an abstract
of the article in the Journal (=16=, 441, 1878). He had discovered a new
law of nature of momentous importance and wide application which is
called the “Phase-Rule” and is expressed by a very simple formula.
The application of this great discovery to chemical theory was delayed
for ten years, partly, perhaps, because it was not sufficiently brought
to the attention of chemists, but largely it appears because it was not
at first understood, since its presentation was entirely mathematical.
It was Rooseboom, a Dutch chemist, who first applied the phase-rule. It
soon attracted profound attention, and the name of Willard Gibbs
attained world-wide fame among chemists. When Nernst, who is perhaps the
most eminent physical chemist of the present time, was delivering the
Silliman Memorial Lectures at Yale a few years ago, he took occasion to
place a wreath on the grave of Willard Gibbs in recognition of his
achievements.
To understand the rule, it is necessary to define the three terms,
introduced by Gibbs, _phase_, _degrees of freedom_ and _component_.
By the first term, is meant the parts of any system of substances which
are mechanically separable. For instance, water in contact with its
vapor has two phases, while a solution of salt and water is composed of
but one. The degrees of freedom are the number of physical conditions,
including pressure, temperature and concentration, which can be varied
independently in a system without destroying a phase. The exact
definition of a component is not so simple, but in general, the
components of a system are the integral parts of which it is composed.
Any system made up of the compound H_{2}O, for instance, whether as ice,
water or vapor, contains but one component, while a solution of salt and
water contains two. Letting P, F, and C stand for the three terms, the
phase-rule is simply