It was thought that the knowledge of the laws relating to the inmost movements of this ether might give us the key to all phenomena, and might make us acquainted with the method in which energy is stored up, transmitted, and parcelled out in its external manifestations.
We cannot study here all the problems which are connected with the physics of the ether.

To do this a complete treatise on optics would have to be written and a very lengthy one on electricity.

I shall simply endeavour to show rapidly how in the last few years the ideas relative to the constitution of this ether have evolved, and we shall see if it be possible without self-delusion to imagine that a single medium can really allow us to group all the known facts in one comprehensive arrangement.
As constructed by Fresnel, the hypothesis of the luminous ether, which had so great a struggle at the outset to overcome the stubborn resistance of the partisans of the then classic theory of emission, seemed, on the contrary, to possess in the sequel an unshakable strength.

Lame, though a prudent mathematician, wrote: "_The existence_ of the ethereal fluid is _incontestably demonstrated_ by the propagation of light through the planetary spaces, and by the explanation, so simple and so complete, of the phenomena of diffraction in the wave theory of light"; and he adds: "The laws of double refraction prove with no less certainty that the _ether exists_ in all diaphanous media." Thus the ether was no longer an hypothesis, but in some sort a tangible reality.

But the ethereal fluid of which the existence was thus proclaimed has some singular properties.
Were it only a question of explaining rectilinear propagation, reflexion, refraction, diffraction, and interferences notwithstanding grave difficulties at the outset and the objections formulated by Laplace and Poisson (some of which, though treated somewhat lightly at the present day, have not lost all value), we should be under no obligation to make any hypothesis other than that of the undulations of an elastic medium, without deciding in advance anything as to the nature and direction of the vibrations.
This medium would, naturally--since it exists in what we call the void--be considered as imponderable.