10 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



in the very same proportion, but the more refrangible (hottest) rays are alone re- 

 tained.* 



20. Now the results of (17), though not what I anticipated as most probable, 

 do not altogether relieve us from some doubt as to the nature of the action of the 

 film of smoke, although those experiments, as well as others which are to be de- 

 tailed in this paper, incline me to M. MELLONI'S opinion, that the smoke acts by 

 its own intimate constitution, and not by its mechanical arrangement. Though 

 I have examined smoky films with a powerful microscope, I have failed in detect- 

 ing the minutely divided particles of carbonaceous matter of which it must un- 

 doubtedly consist. Still the reticulation which fine powder strewed on a surface 

 must form, if it act by the minuteness of the spaces which are left (as in diffraction- 

 experiments on light), must act more intensely when by superposition such reti- 

 culations become more minute and complicated. And it may little matter whether 

 the smoky screens are distinct, and deposited on separate plates mechanically 

 placed in succession, or whether they are accumulated by continued smoking on 

 a single surface. I do not state this with a view to maintain my own original 

 opinion, which I am rather disposed to abandon, and to consider a smoked sur- 

 face, diathermanous, as well as transparent, in the full meaning of the words ; but 

 in extending my experiments to roughened surfaces, I was rather surprised to 

 find that the continued action of furrowing the surface by scratching it with coarse 

 sand-paper, not only diminished the transmission of heat, but increased the specific 

 action on rays of different refrangibility, whilst one would rather have imagined 

 that the action being here due to the destruction of polish, and therefore super- 

 ficial, any exaggeration of the roughness would not have increased the relative 

 diathermancy to rays of low refrangibility. 



21. Conclusive experiments, however, mark an increased sensibility to various 

 kinds of heat by increased roughness. Two plates of salt, marked a and b, having 

 been scored with sand-paper in rectangular directions on both sides, were placed 

 so as to intercept similarly a parallel beam of heat. The difference of the fol- 

 lowing numbers is due to the less degree of roughness of a. 



* Smoked glass is evidently an excessively opaque compound medium, being composed of two 

 parts which absorb opposite ends of the heat spectrum. It is curious to reflect how little the true 



