460 Mr. J. Croll on Conceptions of the Constitution of Matter. 



an atom A strikes a similar atom B at rest, it communicates to 

 B its moving force, yet we believe that the dynamical theory 

 will be found to penetrate deeper into the question than the old 

 theory of inert solidity, as the following considerations may 

 perhaps show. 



If an atom A, perfectly elastic, moving with any given velo- 

 city, strikes a similar atom B at rest, it transfers its entire mo- 

 tion or force to B, and remains at rest itself. But no transfe- 

 rence could possibly take place unless B offered resistance to A. 

 Upon what principle, then, does B offer resistance to the advance 

 of A ? According to the ordinary view B is an inert solid, void 

 of all power to offer any active resistance; but yet it does, never- 

 theless, offer resistance. According to the dynamical view, B 

 is a point offering active resistance to every body which ap- 

 proaches within its sphere. When A reaches the place where 

 resistance or repulsion commences, viz. the surface of B, then 

 A meets with resistance as it advances into B and loses motion 

 in consequence. But the motion thus lost by A is communi- 

 cated to B. This transference goes on till both atoms have the 

 same velocity. At this moment A has transferred to B the half 

 of its moving force. But this condition of things cannot re- 

 main, for A has passed within the repulsive sphere of B (the 

 sphere of its activity) and B within the repulsive sphere of A, 

 and the consequence is, the two atoms must mutually repel each 

 other ; A will therefore still continue to push B forward. B, on 

 the other hand, will continue to push A backwards until A is 

 brought to rest ; after this B will separate from A ; but by this 

 time B's velocity will be equal to that which A originally pos- 



Elasticity on the dynamical theory follows as a necessary 

 consequence. But on the ordinary theory it is wholly incon- 

 ceivable, if it be not contradictory. When A has communi- 

 cated to B the half of its moving force, and the two atoms are 

 moving forward with equal velocity, how, upon the ordinary 

 theory, do they not continue to move side by side with equal 

 velocity ? How can B, an inert solid block, by means of in- 

 ertia, without the exertion of any activity, begin now to act upon 

 A so as to push it backwards and stop its motion ? 



