8 Prof. Thomson on the Dynamical Theory of Heat. 



I have thus, I trust with perfect impartiaUty, gone through 

 all the main experimental points of the author's investigations, 

 and upon the whole I can perceive nothing substantiated vihich is 

 positively irreconcileable with the principle of interference, while 

 the new modifications of the phsenomena here presented, so far 

 as general considerations can be relied on, seem sufficiently con- 

 formable to the undulatory theory : but as to theii- more exact, or 

 quantitative explanation, no definitive opinion can be pronounced, 

 until certain analytical investigations of almost impracticable 

 length and complexity, shall have been gone through, by which 

 alone that theory can be brought into exact and satisfactory com- 

 parison with experiment. 



II. On the Dynamical Theory of Heat, with numerical results 

 deduced from jNIr. Joule's equivalent of a Thermal Unit, and 

 M. Regnault's Obsei-vations on Steam. By William Thomson, 

 31. A., Fellow of St. Peter's College, Cambridge, and Prof essor 

 of Natural Philosophy in the University of Glasgow*. 



Introductory Notice. 

 1. ^IR HUMPHRY DAVY, by his experiment of melting 

 *>^ two pieces of ice by rubbing them together, established 

 the following proposition : — " The phsenomena of repulsion are 

 not dependent on a ])eculiar elastic fluid for their existence, or 

 caloric does not exist." And he concludes that heat consists of 

 a motion excited among the particles of bodies. " To distinguish 

 this motion from others, and to signify the cause of our sensation 

 of heat," and of the expansion or expansive pressure produced 

 in matter by heat, "the name repulsive motion has been adoptedf." 



pendiculav to the edge ; if circular, tending to the centre ; if angular, bi- 

 secting it ; if concave, spreading out, &c. A representation of the appear- 

 ance is given in Plate ii. lig. 8 (p. 155). At p. 190, the Editor adds a me- 

 morandum found among Ur. Hooke's papers, stating, that on March 18, 

 Ifi/^, he "read a discourse" on several new properties of light; which he 

 sums up as follows : — 



" That there is a deflexion of light differing both from reflexion and re- 

 fraction, and seeming to depend on the unequal density of the constituent 

 parts of the ray, whereby light is dispersed from the place of condensation 

 and rarefied or gradually diverged into a quadrant;" 2ndly, that this 

 tnkes place " jierpendicularly to the edge ; " and 3rdly, that " the parts de- 

 flected by the greatest angle are the faintest." 



1 have fully referred to and commented upon Newton's description of the 

 same phicnomenon, conveyed in terms so singularly coincident, in my paper 

 before rcfen-cd to. 



* From the Transactions of the Royal Society of Edinburgh, vol.. \x.part2. 

 Passages added in the present reprint are enclosed in brackets. 



t Prom Davy's first work, entitled " An Essay on Heat, Light, and the 

 Combinations of Light," published in 1799, in " Contributions to Physical 



