We turn, then, inevitably to the menstruum of mysteries, the bank of the insolvent in speculation, to the all-serviceable ether. Ethereal radiations exercise an impulsive power; light-pressure has secured a recognised status among cosmic agencies; and every vibrational system of the luminous type undoubtedly shares the faculty by which light tends to drive minute particles forward along the lines of its propagation. Professor J. J. Thomson, accordingly, considered that but for the drawback of their insufficient velocity, 'very penetrating Röntgen rays' might with advantage be substituted for corpuscular streams as the cause of gravity.[68] They would, in some slight degree, be absorbed by encountered masses, to which they would impart a proportionate amount of momentum. Two bodies mutually shadowing one another would, under such circumstances, be drawn together with a force varying as the inverse square of distance; and if further they absorbed the impinging rays strictly in the measure of their density (as observation shows to be approximately the case), the attraction would increase in the same ratio as the product of their masses. But Röntgen rays travel with the precise velocity of light; they are, in truth, ultra-invisible light; and they must hence be regarded as hopelessly incompetent to explain an influence transmitted at least six million times more rapidly. This was fully admitted by Dr. H. A. Lorentz,[69] who, five years ago, weighed the vibrational hypothesis of gravity in the balance of rigorous calculation, and found it wanting. His equations yielded the unexpected result that, if its postulates were granted, the noted attractions between massive bodies could subsist only on the terms of an incessant waste of electro-magnetic energy. But this is of course inadmissible. The theory involving such a consequence stands self-condemned, to say nothing of the wholly inadequate rate of propagation afforded by it.