applicahle to Le Sage's Theory of Gravitation. 369 



were removed to a, double distance, the imaginary spherical 

 surface described through B with A a« a centre^ would have 

 four times the area ; but the area of the molecule B remain- 

 ing constant, B would therefore only shelter A one fourth the 

 amount it did before ; and accordingly A would be impelled 

 towards B with one fourth the force, — the same being true 

 of B relatively to A. 



10. It is found that gravity is proportional to mass. It 

 must therefore be assumed that, owing to the porosity of 

 bodies, or open structure of the molecules, the graviiic medium 

 (whose particles are extremely minute) can penetrate freely 

 into the interior of bodies, and thus act upon the internal 

 molecules, so that the total effect is proportional (sensibly) to 

 the number of molecules, or gravity is proportional to mass. 

 Of course this could not hold true with an infinite mass ; but 

 it is rigidly demonstrable (by a given degree of porosity) 

 that it could hold true with as near a degree of approximation 

 as experience has shown, and even nearer if necessary. In- 

 dependent physical reasons for inferring this extreme porosity 

 or permeability of matter will be given further on. 



11. My main object in this paper is to meet all possible ob- 

 jections which have been or might be urged against this theory, 

 as, if it be true, it ought to stand against all criticism ; and if, 

 on the other hand, it be erroneous, the sooner it is demon- 

 strated so the better. It may just be remarked, in the first 

 place, that in principle there appears to be no other theory 

 conceivable which at all would satisfy the conditions of gravity. 

 Gravity can be referred to two conceivable causes : — (1) to a 

 motion possessed by the molecules of matter themselves, dis- 

 turbing the equilibrium of pressure of the surrounding medium; 

 (2) to an independent motion of the medium itself acting upon 

 the molecules. The first of these two conditions appears to be 

 inadmissible ; for if gravity were due to a motion of the mo- 

 lecules of matter, then since we can readily modify or interfere 

 with the motion of the molecules of matter (as by heat), we 

 could thereby interfere with gravity. The fact, therefore, that 

 it has been found impossible to interfere with grr.vity, points to 

 a motion in the external medium (which is beyond our control) 

 as the cause. It would be well to keep this fact in view before 

 hghtly regarding Le Sage's theory as a mere hypothesis. To 

 the careful observer it will appear to contain rather the essence 

 of a necessary fact, from the absence of (in principle) any 

 other conceivable cause ; and if the theory can be shown to 

 be a practical one, consistent with admitted mechanical prin- 

 ciples, it will have every condition for acceptance. To us it 

 seems that a closer study of ihe theory only serves to show its 



FhiL Mag. S. 5. Yol. 4. No. 26. Nov. 1877. 2 B 



