134 BULLETIN OF THE 



ever until about the middle of the present century that the con- 

 ception attained a scientific definiteness and currency through the 

 accurate determination of the kinetic or dynamic value of heat. 



The XJndulotory Theory of Light. — Nearly simultaneously with 

 the work of Rumford in the field of heat, the investigations of 

 Dr. Thomas Young, at the beginning of this century, relative 

 especially to the interference of two luminous rays in particular 

 cases, in like manner overthrew the theory of corpuscular emission 

 in the field of light, by demonstrating a destruction or oblitera- 

 tion — quite intelligible as a conflict of wave motion, but entirely 

 inadmissable and unthinkable as a mutual extermination of con- 

 flicting substance.* Through the refined labors of Young, — ad- 

 mirably assisted and re-enforced by the able efforts of his skillful 

 and worthy rival Fresnel, — the varied and complex phenomena of 

 dioptrics were more and more fully brought under the dominion of 

 a rational kinetics. And thus it resulted that the new doctrine of 

 insensible motion obtained from the scientific world a much more 

 rapid and general acceptance in its application to light than in its 

 application to heat. So that it was not unusual some forty or fifty 



by friction in these experiments appeared evidently to be inexhaustible," 

 argued that this product " cannot possibly be a material substance: ") may 

 be said to furnish the first rough approximation to the mechanical equiva- 

 lent of heat. The author estimated the heat produced by a one-horse power 

 as equivalent to that obtained from the burning of nine wax candles, each 

 three-quarters of an inch in diameter ; or to the combustion of a little more 

 than one-third of a pound of wax in two and a half hours. This essay also 

 presents the first suggestion of the mechanical correlation of animal power 

 with heat motion. 



Dr. Young held that Rumford's experiments " appear to aft'ord an un- 

 answerable confutation of the whole of this doctrine : — [that of a ' caloric ' 

 fluid.] - - - If heat is not a substance, it must be a quality ; and this 

 quality can only be motion." {Lectures on Natural Philosophy. 1807: 

 lect. 52: vol. I, pp. 653, 654.) 



" The hypothesis of caloric" says Prof. J. Clerk Maxwell " or the 

 theory that heat is a kind of matter is rendered untenable — first by the 

 proof given by Rumford that heat can be generated at the expense of 

 mechanical work ; and secondly by the measurements of Hirn, which show 

 that when heat does work in an engine, a portion of the heat dismppea7-s." 

 (Theory of Heat. 1872: chap, vill, p. 147.) 



* u Phil. Trans. Roy. Soc. A memoir read July 1, 1802: vol. xcn. p. 

 387 ; and one read November 24, 1803 : vol. xciv. pp. 1-16. 



