ITS SELF-CCWSEKVATION. 155 



At the same time, it should not be forgotten that 

 the attractions are exerted, not between the masses as 

 such, but only between the ultimate force-centers con- 

 stituting the masses. And, finally, since the qualitative 

 diiferences in matter arise from the complexity of group- 

 ing of force-centers, it is evident that the different 

 " kinds " of matter could have no effect whatever upon 

 the intensity of the gravitative pull between any two 

 force-centers. 



Here, again, then, our theory, in the free course of its 

 development, presents a simple, natural explanation of 

 what has long since been experimentally shown to be the 

 fact namely, that gravitation is invariably proportional 

 to the quantity of matter, and is not in the slightest 

 degree influenced by the kind of matter. 



The experiments of Newton rendered this conclusion 

 highly probable, while the more elaborate and delicate 

 experiments of the German astronomer Bessel, in the 

 earlier part of the present century, gave it such complete 

 confirmation as to leave no further room for doubt. * 



We have, then, arrived at a rational account of the 

 fact generalized in the first part of the universal law of 

 gravitation, as that law is usually stated, though it is now 

 evident that its more adequate explanation must be 

 sought through the second part of the law, as may be 

 seen from the law as stated in full. "Every particle of 

 matter in the universe attracts every other particle with a 

 force, whose direction is that of the line joining the two, 

 and whose magnitude is directly as the product of their 

 masses, and inversely as the square of their distance from 



*See Whewell, "Hist, of Inductive Sciences," 3d (N. Y.) Ed., I., 549. 



