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 rôle 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. The fine straight roads traced out by the masters of the last century, and enlarged and levelled by the labour of such numbers of workmen, are now joined together by a crowd of small paths which furrow the field of physics. It is not only because they cover regions as yet little explored where discoveries are more abundant and more easy, that these cross-cuts are so frequent, but also because a higher hope guides the seekers who engage in these new routes. In spite of the repeated failures which have followed the numerous attempts of past times, the idea has not been abandoned of one day conquering the supreme principle which must command the whole of physics.