470 Dr. Hare's Second Letter- to Prof. Faraday. 



similitude between the processes of conduction and radia- 

 tion (21). ■ 



53. Consistently with the hypothesis that electricity is ma- 

 terial, you have shown that an enormous quantity of it must 

 exist in metals. To me it seems equally evident that, agree- 

 ably to the idea that heat is material, there must exist in metals 

 a proportionably great quantity of caloric. The intense heat 

 produced when wires are deflagrated by an electrical discharge, 

 cannot otherwise be consistently accounted for. Agreeably 

 to the same idea, every metallic particle in any metallic mass 

 must be surrounded by an atmosphere of caloric ; since the 

 changes of dimensions, consequent to variations of temperature, 

 can only be explained by corresponding variations in the 

 quantity of caloric imbibed, and in the consequent density of 

 the calorific atmospheres existing in the mass which under- 

 goes these changes*. 



54. Such being the constitution of expansible bodies agree- 

 ably to the hypothesis in question, it seems to me that the pro- 

 cess, by which caloric is propagated through them by conduc- 

 tion, must be extremely different from that by which it is 

 transmitted from one part of space to another by radiation. 

 In the one case, the calorific particle flies like a bullet pro- 

 jected from a gun, but with an inconceivably greater velocity, 

 which is not sensibly retarded by the reflecting or refracting 

 influence of intervening transparent media: in the other case, 



* I subjoin the language which I have held respecting the constitution 

 of expansible solids, during the last twenty years. 



"The expansion of matter, whether solid, liquid, or aeriform, by an in- 

 crease of temperature, may be thus explained : — 



"In proportion as the temperature within any space is raised, there will 

 be more caloric in the vicinity of the particles of any mass contained in the 

 space. The more caloric in the vicinity of the particles, the more of it 

 will combine with them ; and in proportion to the quantity of caloric thus 

 combined, will they be actuated by that reciprocally repellent power, which, 

 in proportion to its intensity, regulates their distance from each other. 



" There may be some analogy between the mode in which each ponder- 

 able atom is surrounded by the caloric which it attracts, and that in which 

 the earth is surrounded by the atmosphere; and as in the latter case, so 

 probably in the former, the density is inversely as the square of the distance. 



" At a height at which the atmospheric pressure does not exceed a grain 

 to the square inch, suppose it to be doubled, and supported at that in- 

 creased pressure by a supply of air from some remote region ; is it not 

 evident that a condensation would ensue in all the inferior strata of the 

 atmosphere, until the pressure would be doubled throughout, so as to be- 

 come at the terrestrial surface 30 poinids instead of the present pressure 

 of 13 pounds? Yet the pressure at the point from which the change would 

 be propagated would not exceed two grains per square inch. 



"In like manner, it may be presumed that the atmospheres of caloric 

 are increased in quantity and density about their respective atoms, by a 

 slight increase in the calorific tension of the external medium." 



