258 KINETIC THEORIES OF GRAVITATION. 



really attracted. As a resultant of all the impressions, the particles 

 would act upon each other in the inverse ratio of the square of the dis- 

 tance, and in the direct ratio of their number, — an action which at once 

 presents a striking analogy with the law of universal attraction." * 



It is not believed that either of these ratios wounl be even approxi- 

 mately attained. It will be observed that in this scheme the setherial 

 vibrations are supposed to exert a precisely opposite action to the undu- 

 lations exhibited in the system of Professor Challis, having their origin 

 on the circumferences of enormous spheres of aether, and being accurately 

 directed to a central point or points, whatever may be the variety of 

 distribution or the changes of position in the material elements. 



Tait. 1864. 



The professor of natural philosophy in the University of Edinburgh, 

 P. G. Tait, has expressed himself with a cautious moderation on the 

 probable origin of gravity, but with a sufficient distinctness to indicate 

 his inclination to a kinetic hypothesis. In an able though somewhat 

 prejudiced and partisan review of "The Dynamical Theory of Heat," 

 published in the North British Eeview for February, 1864, after the 

 very distinct affirmation of the great truth that "natural philosophy 

 is an experimental and not an intuitive science : no a priori reasoning 

 can conduct us definitely to a single physical truth ;" the reviewer thus 

 proceeds to suggest his inductive conclusions : 



"In the physical world we are congnizant of but four elementary or 

 primordial ideas, beside the inevitable Time and Space. They are Mat- 

 ter, Force, Position, and Motion. Of these, motion is simply change of 

 position; and force is recognized as the agent in every change of 

 motion. Till we know what the ultimate nature of matter is, it will be 

 premature to speculate as to the ultimate nature of force ; though we 

 have reason to believe that it depends upon the diffusion of highly at- 

 tenuated matter throughout space." t 



Indefinite as the statement is, the indication that "force" probably 

 depends rather on "highly attenuated matter" than on ordinary gross 

 or sensible matter would appear to be derived from a somewhat meta- 

 physical reason to believe. If the conception of a material connecting- 

 link throughout space may be supposed to rest on a perception of phys- 

 ical fitness or necessity in such a transmitter of energy, this gives no 

 physical reason to believe the origin of force resident in the one form of 

 matter rather than in the other. For whether this "highly attenuated 

 matter throughout space " is supposed to act statically or kinetically, 

 and whether its function be to give or to receive impulses, we are cor- 



* Compter Bendus, vol. lvi, pp. 531, 532. 



t North British Review, February, 1864, vol. xl, art. ii, p. 22 of Am. edition. This 

 essay, with another one on " Energy " in the same periodical, has been republished as 

 a separate treatise, under the title of " Sketch of Thermodynamics." 8vo., Edinburgh, 

 1868. 



