Dr. Hare's Secotid Letter to Prof. Faraday. 471 



it must be slowly imparted from one calorific atmosphere to 

 another, until the repulsion, sustained on all sides, is in equi- 

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

 that metals are bad radiators, while good reflectors*. 



55. In paragraph 25 you allege that conduction of heat 

 differs from electrical induction, because it passes by a very 

 slow process ; while induction is, in its distant influence, si- 

 multaneous with its force at the place of action. How then 

 can the passage of heat by conduction be " a process -precisely 

 like that of radiation" (21), which resembles induction in the 

 velocity with which its influence reaches objects, however 

 remote ? 



56. Although (21) you appeal to the " modern views re- 

 specting radiation and conduction of heat," in order to illus- 

 trate your conception of the contiguity of the particles of 

 bodies subjected to induction, yet (in 25) you object to the 

 reference which I had made to the same views, in order to 

 show that the intensity of electro-polarization could not be 

 inversely as the number of the polarized particles, interposed 

 between the "inductric" surfaces. Let us then resort to the 

 case above suggested, of the influence of the surfaces of the 



* I 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 radiation, is the external stratum. 



" On the other hand, from its porosity, and probably from its not retain- 

 ing caloric within its pores tenaciously as an ingredient in its composition, 

 charcoal oj)poses but little obstruction to the passage of that subtile prin- 

 ciple, v^hen in the radiant form ; and hence its particles may all be simul- 

 taneously engaged in radiating any excess of this principle with which a 

 feeble affinity may have caused them to be transiently united, or in receiv- 

 ing the rays emitted by any heated body, to the emanations from which 

 they may have been exposed. 



" We may account in like manner for the great radiating power of 

 earthenware and wood. 



" For the same reason that calorific rays cannot be projected from the 

 interior of a metal, they cannot enter it when jirojected against it from 

 without. On the contrary, they are repelled with such force as to be re- 

 flected without any perceptible diminution of velocity. Hence the pre- 

 eminence of metallic reflectors. 



" It would seem as if the calorific [)arti(:les which are condensed between 

 those of the metal, repel any other particles of their own nature which may 

 radiate towards the metallic superficies, before actual contact ensues; 

 otherwise, on account of mechanical imperfection, easily discernible with 

 the aid of a microscope, mirrors would not be as efficacious as they are 

 foimd to be in concentrating radiant heat. Their influence, in this respect, 

 seems to result from the excellence of their general contour, and is not 

 proportionably impaired by blemishes." 



