applicable to Le Sages Jlieorij of GracUation. 211 



tion cannot but be regarded as fantastic, and, as we observe, 

 is by no means necessary. The mean length of path of the 

 particles, so far from being comparable to the dimensions of 

 the visible universe, may be but an infinitesimal fraction of 

 the distance of two of its primary components. All we require 

 to admit is that the mean length of path of the particles of the 

 medium is at least as great as the very limited range through 

 which gravity has been observed to act ; or, in order to ex- 

 plain all the observed facts, it is sufficient to admit that the 

 universe is immersed in a gas (or medium constituted accord- 

 ing to the kinetic theory) the mean length of path of whose 

 particles is so adjusted as to cause the minor or secondary por- 

 tions of the universe to gravitate towards each other*. Under 

 the simple conception of a variation in the diameter of the par- 

 ticles of a medium, the mean length of path of the particles 

 (and with it the range of gravity) is capable of adjustment 

 with precision to any range. It would probably be difficult to 

 imagine any more simple condition as a mechanical means to 

 an end than this. 



10. It is a necessary condition to Le Sage's theory (in order 



compared with the mean length of path of an air molecule, to be stationed 

 in the centre of a receptacle containing air ; then this minute observer 

 would notice the air molecules passing continually in streams equally in 

 all directions ; and the observer, not being able to trace the beginning 

 and end of the path of the molecules, would naturally imagine that tlie 

 molecules were being supplied in streams from outside the receptacle, 

 and, observing the continued regularity of the streams equally in all 

 directions, would suppose that there must be some external mechanism 

 supplpng the streams of molecules equally and symmetrically. This 

 is what would be supposed in the absence of the application of the prin- 

 ciples of the kinetic theory to the case. On applying this theory, it is 

 seen that the same result can take place without a supply of air ; and 

 that the adjustment of the streams of air molecules uniformly in all di- 

 rections is automatic, or the inevitable result of dynamical principles. 



* If two particles of a gas be conceived gradually to increase in size 

 and mass, they will gi-adually lose their translatory motion and finally 

 come practically to rest. If now the two enlarged particles be at a dis- 

 tance apart less than the mean length of path of the remaining particles of 

 the gas, these two particles will gravitate towards each other under the 

 dynamic action of the other particles of the gas which pass them in 

 streams (the opposed faces of the two enlarged particles being sheltered 

 from the streams). This condition of things probably occurs when a va- 

 pom- condenses, when the gTadually enlarging water vesicles which happen 

 to be at a distance apart less than the mean length of path of the other 

 molecules of the vapour will be driven together by the dynamic action of 

 these molecules, thus accelerating condensation. Thus possibly effects of 

 " gravit}^ " may be imitated by gaseous matter on molecular scale, as is ob- 

 served in the large scale of the universe. It may be observed that these 

 are simply applications on a smaller scale of the d3"namical principles of 

 Le Sage's theory, and therefore the deductions hold if the premises are 

 admitted to be possible. 



P2 



