FOURTH SERIES. ROUGH SURFACES. \\ 



Here, then, we find the per-centage of transmission raised in every case by a pre- 

 vious transmission through a rough surface. The increased facility of transmis- 

 sion is greater in proportion as the incident heat was more heterogeneous ; dark 

 heat undergoes very little change. It appears also by the last line of the table, 

 that the increased roughness of b compared to a, had enhanced the characteristic 

 effect (analogous to redness for light). 



22. I have made a great many experiments to satisfy myself that the action 

 of all the three media already specified (14) is precisely analogous, and that they 

 actually insulate similar rays by absorption. The following table is a specimen, 

 shewing the increased facility with which rays of heat, from whatever source, are 

 transmitted by smoked rock-salt after previous transmission through the same or 

 other substances. 



TABLE shewing the Per-centage of Transmission by the Smoked Rock-Salt Plate 

 E for heat from different sources, and modified by passing through the fol- 

 lowing Media. 



23. It is very important to consider how this action of rough surfaces may 

 be explained, and whether we have any analogous phenomena in the case of 

 light. Can it be owing to the circumstance that the depolished surface reflecting 

 differently the various kinds of heat, those kinds least copiously reflected per- 

 severe, and form the majority of the transmitted rays ? To this it may be replied, 

 that the intensity of reflection at polished surfaces, is so insignificant at a perpen- 

 dicular incidence for either heat or light,* that were the whole specularly reflect- 

 ed heat, transmitted in the one case, and absorbed in the other, the difference, 

 instead of amounting to 30 per cent, or more, of the incident heat (21), could not 

 exceed 4 per cent. 



24. Arguing from the analogous case of light, I anticipated, on the contrary, 

 that the reflected as well as the transmitted beam, would be more intense from 



cause of the opacity of a film of smoke deposited upon glass was understood at the time that it 

 was quoted as a convincing proof of the immediate radiation of heat through solid bodies. Far 

 from smoke being the untransparent substance supposed (I use the word loosely in applying it to 

 heat), it transmits a quantity of some kinds of heat really surprising, although the thickness of the 

 smoke be considerable. 



* See MELLONI, Ann. de Chimie, Dec. 1835, and my Memorandum on the Intensity of Reflected 

 Heat and Light, Proceedings Royal Society of Edinburgh, p. 254. 



