KINETIC THEORIES OF GRAVITATION. 225 



and destroy it by sudden rarefaction ; and any two^ having ever so 

 small a communication, would quickly and equally intermix."* 



Fanciful as are the considerations which led Herapath to this conclu- 

 sion, it may be doubted whether a better statement of the dynamic 

 theory of heat, and the modern view of gaseous temperature, has been 

 published in the last half century. Certainly none can be found pre- 

 ceding it. The scientific world was not then however prepared by a 

 sufficient induction to fully appreciate this theor3\ 



These views of thermogenetic gravitation were amplified by their 

 author at a later period, and included in an elaborate and excellent 

 treatise on the general principles of physics, published in 1847, in which 

 work they form the concluding portion, or boolj; iv, comprising four 

 sections.t 



Herapath saw very clearly that a theory of molecular collision cannot 

 dispense with resilient impacts ; but he announced the startling paradox 

 that atoms "perfectly hard" would on striking each other, rebound just 

 as though they were elastic. This very difficult thesis is discussed at 

 some length (though certainly not convincingly) in his general work.f 

 in a chapter on " the collision of hard bodies." The conception of a re- 

 pellant propensity in the atoms is of course, excluded by the very s]>irit 

 of the hypothesis. " Only two properties to matter are assumed, namely, 

 inertia and absolute hardness. . . . Our theory deprives the par- 

 ticles of repulsion, or of any active properties, and merely assumes that 

 airs are composed of small particles moving about in all possible direc- 

 tions, and keeping up their state as airs by their mutual collisions and 

 reflections from one another and the sides of the containing vessels. 

 From this simple property, and that of heat consisting in corpuscular 

 motion, the whole known laws of gases are deduced with mathematical 

 rigor." § Unfortunately "two properties" are wholly insufficient either 

 to set or to keep a system of molecules in motion. Matter thus consti- 

 tuted, (with only " two properties,") with any amount of motion super- 

 imposed, could never aiake a cosmos. The " stubborn fact" of elasticity 

 has indeed been the insuperable obstacle and embarrassment of all 

 kinetic schemes of molecular physics. 



" By extending the principles to find the temperatures of the planets, 

 we arrive at an interesting conclusion, namely : supposing them to be 

 all of the density of our earth, we bring out very nearly the amount of 

 gravitation toward each of them which is actually found to exist. 

 Mercury is not included, as our knowledge about him is uncertain." 

 (Introduction, p. xxv.) Mercury however is excluded, because on the 

 assumption that the absolute temperatures of the planets are inversely 

 as their distances from the sun, the temperature of this inner planet is 



* Loco citat., p. 278. 



t Mathematical Physics. By John Herapath. 2 vols. 8vo. London, 1847. 

 tMath. Phys., vol. i, pp. 106-137. Huyghens and Wren had both (a century and a 

 half earlier) maintained the same doctrine. 

 § Math. Phys., Introduction, pp. xvii, xviii. 

 S 15 



