KINETIC THEORIES OF -GRAVITATION. 237 



SEGUIN. 1848. 



Marc Seguio, a French engineer, distinguished as having affirmed in 

 1839, from a study of the locomotive-eDgine, the correlation between 

 heat and " work," and as having estimated the " mechanical equivalent " 

 of heat, labored for many years to establish the unity of the natural 

 forces. It is interesting to observe that like Herapath, he commenced 

 his studies in molecular physics with an inquiry into the nature of heat ; 

 and like him, was led to discard entirely the generally-received theory 

 of a material caloric in favor of the kinetic hypothesis now universally 

 adopted. 



Nearly a quarter of a century before publishing his views on molecular 

 forces generally, Seguin presented to Sir John Herschel a very original 

 and suggestive communication on the probable nature of heat, which 

 was published by the latter in the Philosophical Journal of Edinburgh 

 for 1824. The writer infers from the compressibility of all known sub- 

 stances that their constituent molecules must be at a great relative dis- 

 tance from each other ; and from the characteristic odor of most solids, 

 that the densest and hardest substances are subject to the escape of 

 their surface molecules, or in other words, " are capable of being evap- 

 orated/' From the infinitesimal size of these escaping molecules, they 

 of course elude all known methods of comparison or mechanical appre- 

 ciation. 



"In order to assign to them the condition either of a solid, a liquid, 

 or a gas, it is necessary to suppose the existence and the combination of 

 two forces which are sometimes in equilibrio, and sometimes predominate 

 the one over the other. We shall admit then the supposition that these 

 two forces may be the same as those which regulate the planetary sys- 

 tem, and that the molecules of bodies are subject to circulate round one 

 another, so that each body, though it appears at rest, has really a cer- 

 tain quantity of motion, whose measure will be a function of the mass 

 and the velocity of the molecules in motion. Upon these suppositious 

 it is obvious that during the impact of two bodies, all the quantity of 

 motion which is not employed in giving the body which is struck a 

 motion of translation, will go to augment the quantity of interior motion 

 which it possesses ; and if this motion takes place in circles or ellipses, 

 the parts will recede from the center of attraction, and the body will 

 increase in volume. In this state it will have a tendency to transmit 

 the excess of motion which it possesses to bodies which are near it, or 

 to parts which it will emit in greater number in following the same law. 

 If the quantity of motion is so great that the attraction of the molecules 

 can no longer be in equilibrio with their angular velocities, the body 

 will remain in the gaseous state till it has transmitted to other bodies, 

 the excess of velocity which it possesses."* 



This is a very neat and perspicuous presentation of the dynamical 



* The Edinburgh Philosophical Journal, October, 1824, vol. x, pp. 280-282. 



