KINETIC THEORIES OF GRAVITATION. 277 



necessary to execute precisely its ascertained dynamic function.* This 

 specific and very limited inertia is manifestly inadequate to the indefi- 

 nite accumulation of energy required to give the same proportional 

 tendency between large masses as between small ones. The experiments 

 of Guyot and of Guthrie quite conclusively show that neither in the 

 law of quantity nor in the law of intensity can gaseous vibrations rep- 

 resent even approximately the ascertained facts of gravitation embodied 

 in the second and third propositions. 



But if in dynamic action a gaseous undulation of aether, with its 

 existing co-efficient of inertia, is found to be so palpably inadequate to 

 the known eff"ect, on the other hand the elasticity must be assumed to 

 be incomparably more active than the limit expressed by the actual 

 velocity of a luminous undulation. If its rate of propagation be assumed 

 to be very many millions of times more rapid than that of light, the 

 inertia must be correspondingly reduced ; in short, must be practically 

 nil; and the action must be really kinematic rather than dynamic. 



Failing thus at every point, the hypothesis leaves still more inscruta- 

 ble the origin of the undulation. The center of disturbance is supposed 

 to be the vibrating material element; but the cause of the vibration 

 is never stated. If innate tendency be the answer, never surely was a 

 more mysterious " occult quality " attributed to matter in the history 

 of human excogitations. Either misapprehending altogether, or quite 

 ignoring the origin of thermal vibrations, the kinetic theorist not un- 

 frequently appeals to the dynamical doctrine of heat as the type and 

 the warrant of his assumptions. But in the case of heat, as in every 

 other observed case, motion is a phenomenon to be accounted for; and 

 no physical theory is complete, until in origin, in quantity, and in di- 

 rection, it is accurately explained by a true and sufficient antecedent 

 cause.j 



But granting a prime mover — the immediate operation of a Demi- 

 urgus, if necessary, — how is the initial impulse converted into vibration ? 

 What is the resisting power deflecting the element in motion from that 

 rectilinear direction, which is its first law of action? And by what 

 opposing battledoors (for two are absolutely necessary) does the deflected 

 particle become a shuttlecock I Upon these important questions there 

 is a very remarkable reticence. We know that when a bell is struck 

 with a hammer the moving impulse of a segment of the free edge is 

 resisted by a molecular pressure which (for want of a better name) has 

 been called repulsion ; and that this reaction is in turn resisted by a 

 molecular tension which (for want of a better name) has been called 

 attraction; and that this reciprocating play very rapidly declines as 



* From the actual dynamic energy transmitted by the aether, Sir William Thomson, 

 in 1855, estimated that a cubic foot should weigh not less than the quadrilliontli 

 (1 -^ lO^*) of a grain ; or that the inertia of tether had for its higher limit about one 

 two-thousand-trillionth (| -^ 10-') of that of ordinary air. (L. E. D. Phil. Mag., Jan- 

 nary, 1855, vol. ix, p. 39.) Its loxi-er limit may be one hundred times this amount, or 

 one twenty-trillionth (^ — IQis) of the inertia of air. 



