374 On Le Sage^s Theory of Gravitation. 



total quantity of matter contained in (say a cubic mile) of the 

 medium might be indefinitely small, and yet the particles in- 

 definitely close together, provided the particles are very 

 minute. By a given velocity of the particles of a medium, 

 the uniformity or steadiness of the pressure exerted against 

 matter evidently does not depend on the size of the particles, 

 but on their closeness (which determines the rapidity of suc- 

 cession of the collisions against matter). By a given proximity 

 of particles, therefore, no consequence how minute they may be 

 (and therefore how small the quantity of matter composing the 

 medium), the pressure will remain equally steady. It follows, 

 therefore, that a medium may produce all the uniformity of 

 pressure due to the flow of a continuous fluid, and yet the 

 quantity of matter composing the medium may be indefinitely 

 small. Owing to the frequency of the collisions against matter, 

 due to the close proximity of the particles of the gravific 

 medium and their high velocity, the pressure exerted by the 

 medium is so even and regular as to be imperceptible to the 

 senses, excepting in the effect '■' gravity." The pressure termed 

 '^ gravity " due to the motion of the particles of the gravific 

 medium is no more difficult of realization than the pressure of 

 the air due to the motion of its molecules. To get a true idea 

 of the nature of the gravific medium, the conception of ex- 

 treme closeness of arrangement of the particles, combined with 

 extreme rarity of the medium, must be kept in view. It is 

 easily conceivable, for example, that the particles of a cubic 

 foot of the medium may be in very much closer proximity 

 than the molecules of a cubic foot of lead (from centre to 

 centre), and yet the total quantity of matter contained in a 

 cubic foot of the medium may be less than that contained in 

 a single molecule of lead. 



18. The agent producing gravity must therefore not in any 

 way be looked at (as one might possibly be liable to do at first 

 sight) as representing a prodigious quantity of streams of gross 

 matter flying about, but simply as the quiet imperceptible 

 motion of a relatively very small quantity of excessively finely 

 subdivided matter which produces a perfectly uniform pressure, 

 the energy of each particle by itself being totally imper- 

 ceptible ; or only the resultant effect or pressure is noticed, the 

 inexorable motion of the particles (and the resultant effect 

 '' gravity ") being as incapable of being interfered with as 

 the conservation of energy itself. Surely no more rigid, con- 

 stant and unalterable cause could be conceived of than that of 

 the normal motion of the particles of a medium among them- 

 selves, which, by an inevitable automatic adjustment arrange 

 their motions so as to produce the effects of gravity. "• Gra- 



