However this may be, the edifice thus constructed, being composed of electrons in periodical motion, necessarily grows old.  The electrons become subject to accelerations which produce a radiation towards the exterior of the atom; and certain of them may leave the body, while the primitive stability is, in the end, no longer assured, and a new arrangement tends to be formed.  Matter thus seems to us to undergo those transformations of which the radio-active bodies have given us such remarkable examples.

We have already had, in fragments, these views on the constitution of matter; a deeper study of the electron thus enables us to take up a position from which we obtain a sharp, clear, and comprehensive grasp of the whole and a glimpse of indefinite horizons.

It would be advantageous, however, in order to strengthen this position, that a few objections which still menace it should be removed.  The instability of the electron is not yet sufficiently demonstrated.  How is it that its charge does not waste itself away, and what bonds assure the permanence of its constitution?

On the other hand, the phenomena of gravitation remain a mystery.  Lorentz has endeavoured to build up a theory in which he explains attraction by supposing that two charges of similar sign repel each other in a slightly less degree than that in which two charges, equal but of contrary sign, attract each other, the difference being, however, according to the calculation, much too small to be directly observed.  He has also sought to explain gravitation by connecting it with the pressures which may be produced on bodies by the vibratory movements which form very penetrating rays.  Recently M. Sutherland has imagined that attraction is due to the difference of action in the convection currents produced by the positive and negative corpuscles which constitute the atoms of the stars, and are carried along by the astronomical motions.  But these hypotheses remain rather vague, and many authors think, like M. Langevin, that gravitation must result from some mode of activity of the ether totally different from the electromagnetic mode.

CHAPTER XI

THE FUTURE OF PHYSICS

It would doubtless be exceedingly rash, and certainly very presumptuous, to seek to predict the future which may be reserved for physics.  The role of prophet is not a scientific one, and the most firmly established previsions of to-day may be overthrown by the reality of to-morrow.

Nevertheless, the physicist does not shun an extrapolation of some little scope when it is not too far from the realms of experiment; the knowledge of the evolution accomplished of late years authorises a few suppositions as to the direction in which progress may continue.

The reader who has deigned to follow me in the rapid excursion we have just made through the domain of the science of Nature, will doubtless bring back with him from his short journey the general impression that the ancient limits to which the classic treatises still delight in restricting the divers chapters of physics, are trampled down in all directions.