founded, it cannot be said that the general
dynamics of systems, considered as the study of thermal movements and
variations, are yet as solidly established.

Sec. 5. ATOMISM

It may appear singularly paradoxical that, in a chapter devoted to
general views on the principles of physics, a few words should be
introduced on the atomic theories of matter.

Very often, in fact, what is called the physics of principles is set
in opposition to the hypotheses on the constitution of matter,
particularly to atomic theories. I have already said that, abandoning
the investigation of the unfathomable mystery of the constitution of
the Universe, some physicists think they may find, in certain general
principles, sufficient guides to conduct them across the physical
world. But I have also said, in examining the history of those
principles, that if they are to-day considered experimental truths,
independent of all theories relating to matter, they have, in fact,
nearly all been discovered by scholars who relied on molecular
hypotheses: and the question suggests itself whether this is mere
chance, or whether this chance may not be ordained by higher reasons.

In a very profound work which appeared a few years ago, entitled
_Essai critique sur l'hypothese des atomes_, M. Hannequin, a
philosopher who is also an erudite scholar, examined the part taken by
atomism in the history of science. He notes that atomism and science
were born, in Greece, of the same problem, and that in modern times
the revival of the one was closely connected with that of the other.
He shows, too, by very close analysis, that the atomic hypothesis is
essential to the optics of Fresnel and of Cauchy; that it penetrates
into the study of heat; and that, in its general features, it presided
at the birth of modern chemistry and is linked with all its progress.
He concludes that it is, in a manner, the soul of our knowledge of
Nature, and that contemporary theories are on this point in accord
with history: for these theories consecrate the preponderance of this
hypothesis in the domain of science.

If M. Hannequin had not been prematurely cut off in the full expansion
of his vigorous talent, he might have added another chapter to his
excellent book. He would have witnessed a prodigious budding of
atomistic ideas, accompanied, it is true, by wide modifications in the
manner in which the atom is to be regarded, since the most recent
theories make material atoms i