William Thomson, 1st Baron Kelvin

“ But nothing less than direct observation, proving Kepler's third law Galileo's experiment on bodies falling from the tower of Pisa, Boyle's guinea and feather experiment, and Newton's experiment of the vibrations of pendulums composed of different kinds of substance could give either the idea that gravity is proportional to mass, or prove that it is so to a high degree of accuracy for large bodies and small bodies, and for bodies of different kinds of substance. ”

“ Farther, it is clear that the value of β for a set of equal and similar corpuscules will not be the same after collision with molecules different from them in form or in elastic rigidity, as after collision with molecules only of their own kind. All that is necessary to complete Le Sage's theory of gravity in accordance with modern science, is to assume that the ratio of the whole energy of the corpuscules to the translational part of their energy is greater, on the average, after collisions with mundane matter than after inter-collisions of only ultramundane corpuscules. ”

“ I may merely say that inasmuch as the law of the inverse square of the distance, for every distance, however great, would be a perfectly obvious consequence of the assumptions, were the gravific corpuscules infinitely small, and therefore incapable of coming into collision with one another, it may be extended to as great distances as we please, by giving small enough dimensions to the corpuscules relatively to the mean distance of each from its nearest neighbour. ”

“ If the gravific corpuscules leave the earth or Jupiter with less energy than they had before collision, their effect must be to continually elevate the temperature throughout the whole mass. The energy which must be attributed to the gravific corpuscules is so enormously great, that this elevation of temperature would be sufficient to melt and evaporate any solid, great or small, in a fraction of a second of time. Hence, though outward-bound corpuscules must travel with less velocity, they must carry away the same energy with them as they brought. ”

“ Le Sage held that such collisions are extremely rare; that each collision, even between the ultramundane corpuscules themselves, destroys some energy; [9] that at a not infinitely remote past time they were set in motion for the purpose of k',eping gravitation throughout the world in action for a limited period of time; and that both by their mutual collisions, and by collisions with mundane atoms, the whole stock of gravific energy is being gradually reduced, and therefore the intensity of gravity gradually diminishing from age to age. ”

“ It will be a very interesting subject to examine minutely Le Sage's details on these points, and to judge whether or not the additional knowledge gained by observation since his time requires any modification to be made in the estimate which he has given of the possible degrees of permeability of the sun and planets, of the possible proportions of diameters of corpuscules to interstices between them in the "gravific fluid," and of the possible velocities of its component corpuscules. This much is certain, that if hard indivisible atoms are granted at all, his principles are unassailable ”

“ If the molecules of gases are admitted to be elastic corpuscules, the validity of Clausius' principle is undeniable; and it is obvious that the value of the ratio β must depend upon the shape of each molecule, and on the distribution of elastic rigidity through it, if its substance is not homogeneous. ”

“ The secular diminution of gravity would be more rapid according to this supposition than according to the former, but still might be made as slow as we please by pushing far enough the fundamental assumptions of very small diameters for the cage-bars of the mundane atoms, very great density for their substance, and very small volume and mass, and very great velocity for the ultramundane corpuscules. ”

“ Even for the simplest case, that, namely, of smooth elastic globes, no one has yet calculated by abstract dynamics the ultimate average ratio of the vibrational and rotational, to the translatiorial energy. But Clausius has shown how to deduce it for the corpuscules of any particular gas from the experimental determination of the ratio of its specific heat, pressure constant, to its specific heat, volume constant. ”

“ Le Sage's second fundamental assumption given above, under the title " Constitution of Heavy Bodies," implies sensibly equal permeability in all directions, even in an aeolotropic structure, unless much greater than Jupiter, provided that the atoms are isotropic as to permeability.
A body having different permeabilities in different directions would, if of manageable dimensions, give us a means for drawing energy from the inexhaustible store laid up in the ultramundane corpuscules, thus: First, turn the body into a position of minimum weight ”

“ The ratios of these velocities to those of the planets are several million times greater than the ratios of the gravities of the planets, towards the sun, to the greatest resistance which secular observations allow us to suppose they experience. ”

“ Hat Gott konnen eine animam, deren Natur uns unbegreiflich ist, erschaffen, HO hat er auch konnen eine attractionem universalem materiae imprimiren, wenn gleich solche attractio supra captum ist, da hingegen die Principia Cartesiana allzeit contra captum etwas involviren. ”

“ They are so sparsely distributed, that is to say, their diameters are so small relatively to their mean mutual distances, that not more than one out of every hundred of them meets another corpuscule during several thousands of years. So that the uniformity of their movements is scarcely ever troubled sensibly. ”

“ Loix; dont l'invention et la démonstration, font la plus grande gloire du plus puissant génie qui ait jamais existé: et Cause, qui après avoir fait pendant longtems, l'ambition des plus grands Physiciens ”