The _multa renascentur_ of the poet is verified by the revolutions no less of thought than of speech. Flights of minute material particles have served the turn of theorists often, and in more ways than one, and have as frequently been consigned to discredited oblivion; but they are in vogue once more. George Louis Lesage, of Geneva, devoted sixty-three of the seventy-nine years of his life, which came to an end in 1803, to the elaboration of a mechanical rationale of gravity, first given to the world in the _Transactions_ of the Berlin Academy of Sciences for 1782, and with details of amplification in his _Traité de Physique Mécanique_, edited by Pierre Prévost in 1818.[63] The explanation it offered of molar attractions was by the supposed unceasing impacts of 'ultramundane corpuscles,' speeding in countless numbers and at fabulous velocities, from nowhere everywhere, and thus enforcing the mutual approach of masses of gross matter. This involved the supposition of an infinitesimal screening effect, producing a small inequality in the strength of the bombardment on the sheltered and unsheltered sides of the bodies exposed to it. This inequality, in fact, was taken to be the _causa causans_ of gravity. Yet its production encountered a difficulty. There was required for it a trifling degree of opacity in every kind of matter, while perfect gravitational transparency is asserted by the most delicate observations. Lesage, then, reduced the arrests laid upon his particles to a minimum; one in ten thousand, for instance, might at the utmost be intercepted by the terrestrial globe.[64] Even this insignificant minority, however, would suffice, through the surrender of their momentum to the impeding bodies, to endow them with the noted property of gravitation. Clerk Maxwell urged the objection that the accompanying loss of kinetic energy by the corpuscles should, if transformed into heat, render all gravitating bodies white-hot. But Professor J. J. Thomson holds that the transferred corpuscular energy might, instead of reappearing in thermal form, be converted into some highly penetrative species of radiation capable of escaping unperceived into surrounding space.[65] 'A simple calculation,' he adds, 'will show that the amount of kinetic energy transformed per second in each gramme of the gravitating body must be enormously greater than that given out in the same time by one gramme of radium.' This consequence of Lesage's theory takes one's breath away; the 'fables of the Talmud' seem, by comparison, easy of belief; nevertheless, Lord Kelvin[66] declared it in 1873 to be more complete in the expository sense, and not more arduous in its assumptions, than the kinetic theory of gases.