In what concerns electricity and magnetism, Maxwell abstains from making any choice. It is not that he systematically disdains all that is unattainable by positive methods; the time he has devoted to the kinetic theory of gases sufficiently proves that. I will add that if, in his great work, he develops no complete explanation, he had previously attempted to give one in an article in the Philosophical Magazine. The strangeness and the complexity of the hypotheses he had been obliged to make had led him afterwards to give this up.
The same spirit is found throughout the whole work. What is essential, that is to say what must remain common to all theories, is made prominent; all that would only be suitable to a particular theory is nearly always passed over in silence. Thus the reader finds himself in the presence of a form almost devoid of matter, which he is at first tempted to take for a fugitive shadow not to be grasped. But the efforts to which he is thus condemned force him to think and he ends by comprehending what was often rather artificial in the theoretic constructs he had previously only wondered at.
CHAPTER XIII
Electrodynamics
The history of electrodynamics is particularly instructive from our point of view.
Ampère entitled his immortal work, 'Théorie des phénomènes électrodynamiques, uniquement fondée sur l'expérience.' He therefore imagined that he had made no hypothesis, but he had made them, as we shall soon see; only he made them without being conscious of it.
His successors, on the other hand, perceived them, since their attention was attracted by the weak points in Ampère's solution. They made new hypotheses, of which this time they were fully conscious; but how many times it was necessary to change them before arriving at the classic system of to-day which is perhaps not yet final; this we shall see.
I. Ampere's Theory.—When Ampère studied experimentally the mutual actions of currents, he operated and he only could operate with closed currents.