276 KINETIC THEORIES OF GRAVITATION. 



attraction between the bodies would be impossible ! The scheme there- 

 fore fails not only to satisfy the second condition, (as formerly stated,) 

 but it equally fails to satisfy "indefinitely" the third condition. And 

 this objection lies equally against every variety of the corpuscular hypo- 

 thesis, from that of Bernouilli to that of Leray. 



In the next place, the hypothesis of undulations similarly fails. First, 

 because the motion assumed is not a vera causa. If it be answered 

 that an all-pervading aether and its system of undulations are otherwise 

 established, by the large array of well-observed phenomena shown to 

 be in exact accordance both in quantity and in quality with the mathe- 

 matical deductions from such postulated undulations, the rejoinder is, 

 that such undulations as have thus been established are demonstrably 

 incapable of inducing anything in the slightest degree analogous to 

 gravitation; and therefore the new undulations required have not been 

 otherwise ascertained. And this is a radical defect in all the wave sup- 

 positions. We are not at liberty to assume unknown and undemon- 

 strated actions, entirely different in character from anything which has 

 been actually observed or necessarily inferred. Great as is the credit 

 due to Professor Ohallis, for instance, for his dexterous, persevering, and 

 laborious efforts to develop his theory by the stern logic of mathematics^ 

 the whole demonstration is vitiated by the unwarranted prelimnary 

 assumption of qualities and modes of action in the aether analogous 

 to those of gases; as for example, that the pressure and the density are 

 proportional. Now as a matter of fact, tbe aether is a medium so wholly 

 and so radically dissimilar to any known gas in structure and in proper- 

 ties, that tbe most characteristic features of its luminiferous undula- 

 tions, — rectilinear propagation, with its special incident inflection; select- 

 ive refraction or chromatic aberration ; birefractiou ; and polarization iu 

 its various forms, — find no representative therein ; leadiug a Newton to 

 the reluctant conviction that a gaseous aether could not be the vehicle of 

 light.* The denial therefore, of the verity of fluent undulations in the 

 aether is based upon the skeptical affirmation that beyond experience 

 and safe induction therefrom, we can tell nothing as to the behavior of 

 an unknown agent. 



But if the character of undulation required were established, sec- 

 ondly, it would as in the former case, be entirely insufficient for the 

 purpose. The aether must have the elasticity and the mass or inertia 



* Dr. Young, in referring to Fresnel's discussion of " polarization," says, " it might be 

 inferred that the luminiferous aether pervading all space, and penetrating almost all 

 substances, is not only highly elastic but absolutely solid." (Miscellaneous Works of 

 Thomas Young, London, 3 vols. 8vo, vol. i, No. 18, p. 415.) And Sir John Herscbel re- 

 marks : ■• Every phenomenon of light points strongly to tbe conception of a solid rather 

 than a fluid constitution of tbe luminiferous tether, iu this sense,— that none of its 

 elementary molecules are. to lie supposed' capable of interchanging places, or of bodily 

 transfer to any measurable distance from their own special and assigned localities in 

 'he universe." (Familiar Lectures on Scientific Subjects, London, 1867, lect. vii, p. 

 285.) On tbe other hand, recent mathematical discussions have found some difficulties- 

 e\ en in this hypothesis. 



