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 question of penetrating into the mechanism of molecules, and we were no longer satisfied to look at matter as a mass, the mechanical solutions seemed undetermined and the stability of the edifices thus constructed was insufficiently demonstrated.