260 KINETIC THEORIES OF GRAVITATION. 



motion. We must therefore admit that potential energy can exist in a 

 body or system all whose parts are at rest."* 



The degree of accordance between the logical conclusion and the un- 

 reasoned fact, is a measure of the value of the " singular quasi-meta- 

 physical argument" by which it is deduced. " In dealing with physical 

 science, it is absolutely necessary to keep well in view the all-important 

 principle that nothing can be learned as to the physical world save by 

 observation and experiment, or by mathematical deductions from data so 



obtained."! 



Saig-ey. 1866. 



The following presentation of the theme, though from a litterateur 

 rather than a physicist, is interesting as showing what may be called 

 the percolation of ideas. In a series of essays contributed by Einile 

 Saigey (under the nom de plume of "Edgar Saveney") to the Revue de 

 Deux Mondes for November 1, November 15, and December 15, 1866, 

 the writer proposed to give a popular exposition of "Modern Physics 

 and Recent Views on the Unity of Natural Phenomena." After a brief 

 sketch of the received correlation of forces, he proceeds : " What then 

 is gravity f What is that mysterious force which causes two bodies to 

 attract each other in the direct ratio of their masses and an inverse 

 ratio of their distance "? Two bodies attract each other ! Then matter 

 is not inert ! Is there not then an apparent contradiction between the 

 two terms, matter and inertia ? . . . Now what light is this new 

 theory going to throw upon the principle of gravity? Here is the 

 answer. A substance to which the name of rather has been given is 

 diffused throughout the entire universe. It envelops bodies and pen- 

 etrates into their interstices. The existence of this substance is deduced 

 from a series of proofs, among which luminous phenomena hold the 

 first rank. iEtker is composed of atoms which impinge upon each other 

 and upon neighboring bodies. It forms in this way a universal medium 

 which exerts a constant pressure upon the molecules of ordinary 

 matter. ... It becomes evident that bodies do not owe their 

 gravity to an intrinsic force, but to the pressure of the medium in 

 which they are immersed. The motion of heavy bodies no longer ap- 

 pears to us other than as a transformation of the setherial motions; 

 and gravity henceforth enters into that majestic unity to which we 

 have conducted all physical forces. . . . Are there then strictly 

 speaking, two kinds of matter ? We can hardly conceive it, now that 

 we have resolved everything into motion. In what respect would these 

 two kinds of matter differ ? Would not the one be subject to the same 

 laws of motion as the other "? Can there be two systems of mechanics ? 

 Certainly not ; since there is but one law for motion, there can be but 



* Theory of Heat: by J. Clerk Maxwell, London, 1872, chap, xxii, p. 282. 

 t Nature, 1876, vol. xiv, p. 460. 



