Second Letter from Dr. Hare to Prof. Faraday. 7 



the consequent density of the calorific atmospheres existing in 

 the mass which undergoes these changes.* 



Such being the constitution of expansible bodies, agreeably 

 to the hypothesis in question, it seems to me that the process, 

 by which caloric is propagated through them by conduction, 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 cannon ball, with an inconceivably 

 greater velocity, which is not sensibly retarded by the reflecting 

 or refracting influence of intervening transparent media : in the 

 other case it must be slowly imparted from one calorific atmos- 

 phere to another, until the repulsion sustained on all sides is in 

 equilibrio. It is in this way that I have always explained the 

 fact that metals are bad radiators, while good reflectors.f 



* I subjoin the language which I have held respecting the constitution of expan- 

 sible solids, during the last twenty years. 



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

 temperature, may be thus explained. 



" In proportion as the temperatui-e 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 may abound in the vicinity of the particles, the more of it will com- 

 bine 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 ponderable atom 

 is surrounded by the caloric which it attracts, and that in which the earth is sur- 

 rounded 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 increased 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 become at the terrestrial surface, 30 lbs. instead of the 

 present pressure of 15 lbs.? Yet the pressure at the point from which the change 

 would be propagated, would not exceed two grains per square inch. In like man- 

 ner, 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 ten- 

 sion of the external medium." 



1 1 will here quote the rationale which has been given in my lectures for the last 

 twenty years. '• Metals appear to consist of particles so united with each other, or 

 with caloric, as to leave no pores through which radiant caloric can be projected. 

 Hence the only portion of any metallic mass which can yield up its rays-by radia- 

 tion, is the external stratum. 



" On the other hand, from its porosity, and probably also from its not retaining cal- 

 oric within its pores tenaciously as an ingredient in its composition, charcoal oppo- 

 ses but little obstruction to the passage of that subtile principle, when in the radi- 

 ant form ; and hence its particles may all be simultaneously engaged in radiating 



