Kinetic Theories of Gravitation. William Bower Taylor

      Lesage, 1750

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      Some fifteen years later, another bold scheme of universal impact or pressure, designed to explain and supersede "attraction," was conceived by Georges-Louis Lesage, a French-Swiss physicist and mathematician. By means of an infinite number of "ultramundane corpuscles" of transcendent minuteness and velocity, traversing space in straight lines in all directions, atoms and masses of matter are impelled together differentially in the lines of their reciprocal mechanical shadows, or in the direction in which the rectilinear impulses of the " corpuscles" are uncounteracted by opposing ones, from the intervention of other atoms or masses.

To quote Arago's exposition of the theory, "A single body placed in the midst of such an ocean of moving corpuscles would remain at rest, since it would be equally impelled in every direction. On the other hand, two bodies ought to advance toward each other, since they would form a mutual screen, as their opposed surfaces would no longer be hit in the direction of the line joining them by the ultramundane corpuscles, and there would then exist currents, the effect of which would no longer be neutralized by opposite currents. Moreover, it will be readily seen that two bodies plunged into such ' gravitation fluid ' would tend to approach each other with a force varying inversely as the square of the distance."^[1]

      Although this scheme presents merely the exchange of one incomprehensible agent for another, it is perhaps one of the most ingenious attempts ever made to substitute the conception of primaeval motion for that of static tension.

      Lesage was only twenty-three years old, when in 1747 he first devised [218] his system of nature; and it is related in his biography that in the enthusiasm of his supposed discovery of so august a secret, he cried out, in the words of the Syracusian Sage, "eureka! eureka" and though late at night, he immediately wrote to his father, under date of January 15, 1747, "Eureka! eureka! Never have I felt such satisfaction as at this moment, in which I have just succeeded in explaining completely, by the simple laws of rectilinear movement, the principle of universal gravitation ! "

      His first production (written in unsuccessful competition for a prize of the Academy) was an Essai sur l'origine des forces mortes, in 1740. This memoir was principally occupied with his mechanical basis of gravitation. Lesage wrote much, and published little. A memoir by him entitled Essai de Chimie Mecanique, which explained the phenomena of elective affinities by currents of ultramundane corpuscles of unequal size, was crowned by the Academy of Rouen in 1758. Another essay by him entitled Loi qui comprend toutes les Attractions et Repulsions, was published in the Journal des Savants, for April, 1764. Eighteen years later, he wrote a dissertation entitled Lucrèce Neutonien, more fully developing his system, and comprising a response to the objections which had been urged against it. This treatise was published in the Memoires de l'Academic de Berlin, for 1782. He also left a Traite des Corpuscles ultramondaines, alluded to with high praise by Prevost in his account of Lesage's life and works, but which appears never to have been published.

      For more than fifty years did Lesage, with unwavering faith, proclaim his doctrine of what he called the " gravific fluid," and urge upon his contemporaries its adoption ; but without success. The scheme has been rejected by intelligent physicists and astronomers as valueless in dealing with the complex facts of nature.

Of the six requirements heretofore specified, it will be found to satisfy but two—the first and the third. So far from fulfilling for example the second condition, (the ratio of mass,) on which Lesage himself most confidently expatiated, it can apparently give no true account of the behavior of a series of atoms placed in a line between two outer ones. The author supposed that he had covered the ground by the assumption that material atoms are so exceeding small in comparison with their interspaces that but few of the flying " corpuscles" will encounter the atoms. Professor Tait, of the University of Edinburgh, has remarked : " It is necessary also to suppose that particles and masses of matter have a cage-like form, so that enormously more corpuscles pass through them than impinge upon them ; else the gravitation action between two bodies would not be as the product of their masses."^[2] While this supposition fails notably to give a satisfactory mathematical representation of the observed facts, (on any assignable ratio of impact to percolation,) it is of course quite inadmissible with respect to atoms themselves. [219] Indeed, if the atoms of matter are porous or penetrable to the "ultramundane corpuscles," the third condition will remain unsatisfied.

      This corpuscular system of course entirely ignores the fourth condition of the problem, and its fundamental postulate stands in direct opposition to the fifth condition. It is certainly impossible, on any quantitative assumption or numerical estimate whatever, to represent by this scheme the earth's residual gravitation toward the sun during an eclipse of the moon.

Professor J. Clerk Maxwell, discussing the theory of Lesage, observes that if the number of ultramundane corpuscles arrested by our earth is by supposition much less than the number arrested by the sun, " the proportion of those which are stopped by a small body, say a one-pound shot, must be smaller still in an enormous degree, because its thickness is exceedingly small compared with that of the earth. Now the weight of the ball, or its tendency toward the earth, is produced according to this theory, by the excess of the impacts of the corpuscles which come from above, over the impacts of those which come from below and have passed through the earth. Either of these quantities is an exceedingly small fraction of the momentum of the whole number of corpuscles which pass through the ball in a second, and their difference is a small fraction of either, and yet it is equivalent to the weight of a pound. … . Now the energy of a moving system is half the product of its momentum into its velocity. Hence the energy of the corpuscles which by their impacts on the ball during one second, urge it toward the earth, must be a number of foot-pounds equal to the number of feet over which a corpuscle travels in a second, that is to say, not less than thousands of millions. But this is only a small fraction of the energy of all the impacts which the atoms of the ball receive from the innumerable streams of corpuscles which fall upon it in all directions. Hence the rate at which the energy of the corpuscles is spent in order to maintain the gravitating property of a single pound is at least millions of millions of foot-pounds per second. What becomes of this enormous quantity of energy ? … The explanation of gravitation falls to the ground if the corpuscles are like perfectly elastic spheres, and rebound with a velocity of separation equal to that of approach. If on the other hand they rebound with a smaller velocity, the effect of attraction between the bodies will no doubt be produced ; but then we have to find what becomes of the energy which the molecules have brought with them but have not carried away. If any appreciable fraction of this energy is communicated to the body in the form of heat, the amount of heat so generated would in a few seconds raise it, and in like manner the whole material universe, to a white heat."^[3]

      Hence the energy expended by the ultramundane corpuscles in giving motion to material masses must be so much abstracted from their aggregate store of velocity ; and from the constantly-increasing [220] number of such corpuscles which must thus be more or less "spent" in fulfilling their appointed function, it follows that the total activity of bombardment on matter cannot be as vigorous now as it was a million years ago, and must be still less vigorous a million years hence ; all which is contrary to the unchangeable continuity of gravity affirmed by our sixth condition.

As has been well remarked by an able anonymous writer in the North British Review, "The attraction of gravitation is not as the surface of the bodies, but as their mass. Lesage had therefore to suppose his solid bodies not solid, but excessively porous, built up of molecules like cages, so that an infinite number of atoms went through and through then), allowing the last layer of the sun or earth to be struck by just as many atoms as the first, otherwise clearly the back part of the sun and earth would gravitate more strongly than the front or nearer sides, which would be struck only by the siftings of the previous layers of matter. This notion involves a prodigious quantity of material in the shape of flying atoms, where we perceive no gross matter, but very little material in solid bodies, where we do find gross matter ; and it further requires