eep and philosophical reason, when,
in the fine discourse pronounced by him at the opening ceremony of the
Congres de Physique in 1900, he exclaimed: "The mind of Descartes
soars over modern physics, or rather, I should say, he is their
luminary. The further we penetrate into the knowledge of natural
phenomena, the clearer and the more developed becomes the bold
Cartesian conception regarding the mechanism of the universe. There is
nothing in the physical world but matter and movement."

If we adopt this conception, we are led to construct mechanical
representations of the material world, and to imagine movements in the
different parts of bodies capable of reproducing all the
manifestations of nature. The kinematic knowledge of these movements,
that is to say, the determination of the position, speed, and
acceleration at a given moment of all the parts of the system, or, on
the other hand, their dynamical study, enabling us to know what is the
action of these parts on each other, would then be sufficient to
enable us to foretell all that can occur in the domain of nature.

This was the great thought clearly expressed by the Encyclopaedists of
the eighteenth century; and if the necessity of interpreting the
phenomena of electricity or light led the physicists of last century
to imagine particular fluids which seemed to obey with some difficulty
the ordinary rules of mechanics, these physicists still continued to
retain their hope in the future, and to treat the idea of Descartes as
an ideal to be reached sooner or later.

Certain scholars--particularly those of the English School--outrunning
experiment, and pushing things to extremes, took pleasure in proposing
very curious mechanical models which were often strange images of
reality. The most illustrious of them, Lord Kelvin, may be considered
as their representative type, and he has himself said: "It seems to me
that the true sense of the question, Do we or do we not understand a
particular subject in physics? is--Can we make a mechanical model
which corresponds to it? I am never satisfied so long as I have been
unable to make a mechanical model of the object. If I am able to do
so, I understand it. If I cannot make such a model, I do not
understand it." But it must be acknowledged that some of the models
thus devised have become excessively complicated, and this
complication has for a long time discouraged all but very bold minds.
In addition, when it became a ques