46 Dr. J. Croll on Le Sage's Theory of Gravitation. 



have been fully met. It is a necessary condition of Le Sage's 

 theory, in order that gravity may be proportional to mass, 

 that the total volume of the free spaces in a substance in the 

 form of interstices between the molecules must be great com- 

 pared with the total volume of matter contained in the mole- 

 cules themselves. This condition of free interstices Mr. Preston 

 considers to be satisfied by assuming the molecules to be small 

 relative to their mean distances. 

 r* Were we at liberty to make any assumptions we choose in 

 reference to the smallness of the molecules of matter and their 

 distance apart, we might be able to satisfy the conditions of 

 Le Sage's theory as to mass ; but this we are not at liberty to 

 do. Modern physics has enabled us to determine, at least 

 roughly, the size of the ultimate molecules of matter and also 

 their distance apart. This subject has recently been investi- 

 gated by Sir William Thomson, the details of which will be 

 found in a remarkable paper in ' Nature,' vol. i. p. 551. Sir 

 William says the diameter of the molecule cannot be less than 

 500,000,000 °^ a C8n timetre. The number of molecules in a 

 cubic centimetre of a liquid or a solid may, he says, be from 

 3 x 10 24 to 3 x 10 26 . This gives the distance from centre to 

 centre of two consecutive molecules to be from , An * to 



. 140,0D0,000 



460,000,000 °* a centimetre. -Now, if we take the mean of 

 these two values, we have 30Q)0 q 0)000 °f a centimetre for the 

 distance between the centres. The mean spaces between the 

 molecules are therefore less than the diameter of the molecules 

 themselves. Under this condition of things, it must be abso- 

 lutely impossible that a gravific particle, even though it were 

 infinitely small, could penetrate to the extent of a thousandth 

 part of a centimetre into the interior of a body without having 

 its motion stopped by coming into collision with a molecule. 

 Le Sage's theory appears therefore to be utterly irreconci- 

 lable with Sir William's conclusions regarding the size of the 

 material molecule. But even supposing we were to assume, 

 what we are hardly warranted to do, that the molecules are 

 10,000 times smaller, and their distance apart 10,000 times 

 greater than Sir William Thomson concludes, still this would 

 not assist the theory. The gravific particles would then, no 

 doubt, penetrate a little further into the interior of a body ; 

 but beyond a few feet, or perhaps a few inches, no particle 

 could go. 



towards the acting mass, and (2) that its intensity diminishes as the square 

 of the distance. But some of Mr. Taylor's objections have already been met 

 by Mr. Preston in his memoir ; beside, one or two of Mr. Taylor's funda- 

 mental postulates seem doubtful. 



