500 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



Thus far the science of Heat is made to rest wholly upon the 

 facts of experiment, and is independent of any hypothesis respect- 

 ing the molecular constitution of bodies. The dynamical theory of 

 Heat, however, has materially aided in establishing true physical 

 conceptions of the nature of heat. The old hypothesis of caloric, as 

 a separate substance, was indeed rendered improbable by the 

 experiments of Eumford and Davy, and by the reasonings of 

 Young ; but it continued to hold its ground, and was interwoven 

 into the language of science. It is now clearly shown to be self- 

 contradictory ; and to lead to the result, that the amount of heat 

 in the universe may be indefinitely augmented. On the other 

 hand, the identification of radiant heat with light, and the esta- 

 blishment of the wave-theory, left little doubt that heat consisted in 

 a vibratory movement either of the molecules of bodies, or of the 

 ether within them. Still, the relation of heat to bodies, and the 

 phenomena of conduction, indicate a mechanism of a more compli- 

 cated kind than that of light, and leave ample room for further 

 speculation. 



The only mechanical hypothesis (so far as I am aware) which 

 is consistent with the present state of our knowledge of the pheno- 

 mena of heat, is the theory of molecular vortices of Mr. Bankine. 

 In this theory all bodies are supposed to consist of atoms, composed 

 of nuclei surrounded with elastic atmospheres. The radiation of light 

 and heat is ascribed to the transmission of oscillations of the nuclei ; 

 while thcrmometric heat is supposed to consist in circulating cur- 

 rents, or vortices, amongst the particles of their atmospheres, where- 

 by they tend to recede from the nuclei, and to occupy a greater 

 space. From this hypothesis Mr. Eankine has deduced all the 

 laws of thermo-dynamics, by the application of known mechanical 

 principles. He has also, from the same principles, deduced rela- 

 tions (which have been confirmed by experiment) between the 

 pressure, density, and absolute temperature of elastic fluids, and 

 between the pressure and temperature of ebullition of liquids. 



The dynamical theory of heat enables us to frame some conjec- 

 tures to account for the continuance of its supply, and even to 

 speculate as to its source. The heat of the Sun is dissipated and lost 

 by radiation : and must be progressively diminished unless its 

 thermal energy be supplied. According to the measurements of 

 M. Pouillet, the quantity of heat, given out by the Sun in a year, 

 is equal to that which would be produced by the combustion of a 



