PHILOSOPHICAL SOCIETY OF WASHINGTON. 133 



quality, have on their own declaration no right to the use of either 

 term in considering any physical problem. 



Were the examination to stop here, it might appear that the only 

 difference between the dynamist and the kinematist is that the 

 former — failing to find any satisfactory explanation of certain habi- 

 tudes of matter, despairs of deeper insight and accordingly seeking 

 no further, accepts the conclusion that these are insoluble ; while 

 the kinematist more hopeful, has an abiding faith that the same 

 processes which have so successfully (or at least so largely) deciph- 

 ered the riddles of light, of heat, of gaseous constitution, may be 

 expected in time to resolve these other enigmas though they be not 

 yet expounded. It is necessary therefore to go back still further 

 and examine the character of this induction, by a cursory review 

 of the postulates of the mechanical theory of light, of heat, and of 

 the kinetics of discrete molecules. 



2. The Theory of Molecular Kinetics. 



In the last century both light and heat were generally regarded 

 as material emanations ; the former, of radiant corpuscles, the 

 latter, of a peculiarly rare and penetrating fluid. Earlier kinetic 

 hypotheses of these so-called " imponderables " — however ingeni- 

 ous — were not supported by a sufficient induction from observed 

 facts to justly entitle them to unqualified acceptance. And the 

 doubts and difficulties suggested by the speculations of Newton 

 were a striking illustration of his recognized sagacity ; notwith- 

 standing the occasional censures of modern popular lecturers, 

 trumpeting their own superior wisdom. 



The Vibratorxj Theory of Heat. — The fluid or " caloric " theory 

 of heat (though often questioned or opposed) was first decisively 

 overthrown at the close of the century by Benjamin Thompson, an 

 expatriated American, better known as Count Rumford, whose ex- 

 periments unescapably demonstrated the resolution of heat into an 

 intestine motion, by the fact of its interminable generation in fric- 

 tion through the agency of continued motion.* It was not how- 



* Phil. Trans. Roy. Soc. 1798: vol. lxxxiii, pp. 80-102. This admi- 

 rable memoir read before the Koyal Society of London, January 25, 1798, 

 (in which Eumford — from the fact " that the source of heat generated 



