HISTORY OF DYNAMICAL THEORY OF HEAT. 333 



extension of this experimental process to all modes of heat-production 

 that constituted the great work of Joule, to be described hereafter. 



But if Davy thus failed to render his experiments truly conclusive 

 against the materiality of heat, his subsequent observations showed 

 that individually his perceptions were most clear and definite. 



Heat, ultimately, he conceived to depend upon molecular motion 

 calling this the repulsive motion and to produce an effect exactly 

 opposite to that of cohesion. The action of this motion in altering 

 the state of aggregation, he interpreted essentially as is the custom 

 now, and spoke of temperatures as indicating the relative quantities 

 of repulsive motion in the same substance. He also mentioned three 

 modes in which this motion might be increased : 



" 1. By the transmutation of mechanical into repulsive motion, that is, by 

 friction or percussion. In this case the mechanical motion lost by the masses 

 of matter in friction is the repulsive motion gained by their corpuscles. 



" 2. By the motion of chemical combinations of decomposition. 



" 3. From the communicated repulsive motion of bodies in apparent contact, 

 that is, by conduction simply. And subsequently he generalized this statement 

 in the dictum : l 



"The immediate cause of the phenomena of heat, then, as Lavoisier long ago 

 stated, is motion, and the laws of its communication are precisely the same as 

 the laws of the communication of motion." 



These essays of Rumford and Davy failed to produce, with a few 

 rare exceptions, any perceptible effect upon the scientific opinions of 

 their contemporaries. There would seem to have prevailed at this 

 time a remarkable incapacity to appreciate the importance of experi- 

 ments whose indications were opposed to preconceived ideas, and 

 an antipathy to engage in unfamiliar issues; and the same distrust 

 and indifference which so deadened the brilliance of Fresnel's immor- 

 tal work in France proved quite effectual in deferring for the time 

 the discoveries which might otherwise have followed the immediate 

 development and experimental prosecution of this theory. Whatever 

 interest was awakened seems to have been, for the most part, dis- 

 played in the petty, irrelevant objections, and misstatements even, 

 brought against their methods of experiment and observed results : 

 and the injustice of which, when not apparent, might have been easily 

 exposed by a careful repetition or extension of these same determina- 

 tions. 



Dr. Thomas Young, however, in his " Lectures on Natural Philos- 

 ophy," delivered at the Royal Institution, and published in 1807, 2 



lu Elements of Chemical Philosophy," 1812. Complete Works, vol. iv., p. 66. The 

 laws of motion here referred to were those of Newton, especially the third, application to 

 molecular magnitudes being included, and the modifications introduced by the new facts 

 as to the effect cf friction understood ; for, " in Newton's day, and long afterward, it was 

 supposed that work was absolutely lost by friction." (Thomson and Tait, " Natural 

 Philosophy," p. 108.) 



' 2 "Lectures on Natural Philosophy," vol. i., p. 653, etseq. 



