80 



Professor Forbes's Researches on Heat. 



Here, then, we find the per-centage of transmission raised in 

 every case by a previous transmission through a rough sur- 

 face. The increased facility of transmission is greater in pro- 

 portion 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 red- 

 ness for light). 



22. I have made a great many experiments to satisfy my- 

 self 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 speci- 

 men, showing 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 sub- 

 stances. 



Table showing the Per-centage of Transmission by the Smoked 

 Rock-Salt Plate E for heat from different sources, and mo- 

 dified by passing through the following Media. 



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

 surfaces may be explained, and whether we have any analo- 

 gous phasnomena 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 

 persevere, and form the majority of the transmitted rays ? To 

 this it may be replied, that the intensity of reflexion at po- 

 lished surfaces is so insignificant at a perpendicular incidence 

 for either heat or light*, that were the whole specularly re- 

 flected 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 such a surface, as it is well 

 known that polish becomes more specular for rays of light con- 



* See Melloni, Ann. de Chimie, Dee. 1835, and my Memorandum on 

 the Intensity of Reflected Heat and Light, Proceedings of the Royal So- 

 ciety of Edinburgh, p. 254. 



