er when submitted to periodical variations of
temperature, have likewise afforded verifications of the theory
propounded by M. Duhem.

In this theory, the representative system is considered dependent on
the temperature of one or several other variables, such as, for
example, a chemical variable. A similar idea has been developed in a
very fine set of memoirs on nickel steel, by M. Ch. Ed. Guillaume. The
eminent physicist, who, by his earlier researches, has greatly
contributed to the light thrown on the analogous question of the
displacement of the zero in thermometers, concludes, from fresh
researches, that the residual phenomena are due to chemical
variations, and that the return to the primary chemical state causes
the variation to disappear. He applies his ideas not only to the
phenomena presented by irreversible steels, but also to very different
facts; for example, to phosphorescence, certain particularities of
which may be interpreted in an analogous manner.

Nickel steels present the most curious properties, and I have already
pointed out the paramount importance of one of them, hardly capable of
perceptible dilatation, for its application to metrology and
chronometry.[13] Others, also discovered by M. Guillaume in the course
of studies conducted with rare success and remarkable ingenuity, may
render great services, because it is possible to regulate, so to
speak, at will their mechanical or magnetic properties.

[Footnote 13: The metal known as "invar."--ED.]

The study of alloys in general is, moreover, one of those in which the
introduction of the methods of physics has produced the greatest
effects. By the microscopic examination of a polished surface or of
one indented by a reagent, by the determination of the electromotive
force of elements of which an alloy forms one of the poles, and by the
measurement of the resistivities, the densities, and the differences
of potential or contact, the most valuable indications as to their
constitution are obtained. M. Le Chatelier, M. Charpy, M. Dumas, M.
Osmond, in France; Sir W. Roberts Austen and Mr. Stansfield, in
England, have given manifold examples of the fertility of these
methods. The question, moreover, has had a new light thrown upon it by
the application of the principles of thermodynamics and of the phase
rule.

Alloys are generally known in the two states of solid and liquid.
Fused alloys consist of one or several solutions of the component
metals