226 KINETIC THEORIES OF GRAVITATION. 



found to be too high to satisfy the conditions of the calculation.* If 

 " the amount of gravitation toward each " planet is at all indicated by 

 the relative distance-periods of their satellites, it is very clear that they 

 cannot have the same density. 



It might be expected that with the range of temperatures at our com- 

 mand, the influence of heat on attraction could be subjected to the test 

 of direct experiment. It is admitted that " We have no distinct evi- 

 dence of attraction being either augmented or lessened by heat."t 



The radical defect of this ingenious application of the differential of 

 heat-motion as the impelling force of gravity lies in the fallacy that 

 any pressure-differences would, under the circumstances, result from 

 temperature-differences. Our author says : (i In Newton's day the 

 notion of a fluid which had no visible tendency to one part of space 

 more than to another, keeping up an equilibrium with itself, and yet 

 able to press heavier on one side of a body within it than on the other, 

 was quite enough to gain incredulity."! Nor is it easy to perceive how 

 the notion is made more credible in our day. The rarefaction of a free 

 gas by heat is the direct effect of its increased elastic tension or pressure, 

 and the two are proportional. In other words, if upon the planetary 

 hemisphere exposed to the sun there were fewer impacts of gaseous 

 molecules in a unit of time than on the outer or night hemisphere, 

 these impacts would have a correspondingly higher velocity, so that 

 the whole moment of impulse (or pressure) on the two sides would be 

 precisely equal. 



It is doubtful whether this hypothesis (even supposing it operative) 

 could really satisfy any of the six conditions heretofore propounded. 

 With regard to the second postulate, it is evident that the mass of the 

 attracting body cannot determine the quantity of attractive action, if 

 heat be the efficient cause. This is very frankly conceded by Herapath, 

 who says of the mass ratio : " This law has been proved experimentally 

 by Sir Isaac Newton ; but though this be true, the converse case does 

 not according to our principles hold good, namely that the attractive 

 forces of bodies are directly proportional to their quantities of matter. 

 Our principles do not therefore corroborate Newton's third law of 

 motion, respecting the equality of action and reaction in attracting 

 forces; for by our theory, a body might by the agencyiof the fluid 

 medium, be impelled toward another, without any reciprocal action ; 

 which is by no means surprising if we consider attraction not to be an 

 inherent or essential property of matter, but merely the action of a 

 third body."§ The sufficient answer to all which is, that not only is it 

 unconfirmed by any experimental research, but all experience contra- 

 dicts the assumption. 



* Math. Phys., vol. ii, p. 31n. 



t Math. Phys., Introduction, p. xv. 



J Loco citat., Introduction, p. xxxvi. 



§ Annah of Philosophy, new series, vol. i, p. 411 ; and Math. Phys., vol. i, p. 9. 



