Prof. Thomson un the Dynamical Theory of Heat. 9 



2. The dynamical theory of heat, thus established by Sir 

 Humphry Davy, is extended to radiant heat by the discovery 

 of phsenomena, especially those of the polarization of radiant 

 heat, which render it excessively probable that heat propagated 

 through "vacant space," or through diathermanic substances, con- 

 sists of waves of transverse vibrations in an all-pervading medium. 



3. The recent discoveries made by Mayer and Joule*, of the 

 generation of heat through the friction of fluids in motion, and 

 by the magneto-electric excitation of galvanic currents, would 

 either of them be sufficient to demonstrate the immateriality of 

 heat ; and would so afford, if required, a perfect confii-mation of 

 Sir Humphry Davy's views. 



4. Considering it as thus established, that heat is not a sub- 

 stance, but a dynamical form of mechanical effect, we perceive 

 that there must be an equivalence between mechanical work and 

 heat, as between cause and effect. The first published statement 

 of this principle appears to be in Mayei-'s Bemerkungen ilber 

 die Krafts der unbeiebten Natwf, which contains some correct 

 views regardmg the mutual convertibility of heat and mechanical 

 effect, along with a false analogy between the approach of a 

 weight to the earth and a diminution of the volume of a conti- 

 nuous substance, on which an attempt is founded to find nume- 

 rically the mechanical equivalent of a given quantity of heat. In 

 a paper published about fourteen months later, " On the Calorific 

 Effects of Magneto-Electricity and the Mechanical Value of 

 HeatJ," Mr, Joule of Manchester expresses very distinctly the 

 consequences regarding the mutual convertibdity of heat and 

 mechanical effect which follow from the fact, that heat is not a 

 substance but a state of motion ; and investigates on unquestion- 

 able principles the " absolute numerical relations," according to 

 which heat is connected with mechanical power ; verifying expe- 

 rimentally, that whenever heat is generated from purely mecha- 

 nical action, and no other effect produced, whether it be by 

 means of the friction of fluids or by the magneto-electric excita- 



and Medical Knowledge, i)rincipally from the West of England, collected 

 by Thomas Beddoes, M.D.," and republished in Dr. Da^7's edition of his 

 brother's collected works, vol. ii. Loud. 1836. 



* In May 1842, Mayer announced iu the Annalen of Wbhler and Liebig, 

 that he had raised the temi)erature of water from 12° to \:i° Cent, by agi- 

 tating it. In August 184^, Joule announced to the British Association, 

 "That heat is evolved by the })assage of water through narrow tubes ; " 

 and that ho had " obtained one degree of heat jjcr lb. of water from a me- 

 chanical force capable of raising 7/0 lbs. to the height of one foot;" and 

 that heat is generated when work is si)ent in turning a magneto-electric 

 machine, or an electro-magnetic engine. (See his jiaper " On the Calorific 

 Effects of Magneto-Electricity, and on the MecJianical Value of Heat."— 

 Phil. Mag., vol. xxiii. 184.J.) 



t Annalen of Wiihler and Liebig, May 1842. 



i British Association, August 1843; and fhil. Mag., Sept. 1843. 



