Fourth Series. — Rough Surfaces. 79 



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 detecting 

 the minutely divided particles of carbonaceous matter of which 

 it must undoubtedly consist. Still the reticulation which fine 

 powder strewed on a surface must form, if it act by the mi- 

 nuteness of the spaces which are left (as in diffraction-expe- 

 riments on light), must act more intensely when by superpo- 

 sition such reticulations become more minute and complicated. 

 And it may little matter whether the smoky screens are di- 

 stinct, 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 dis- 

 posed to abandon, and to consider a smoked surface, diather- 

 manous, as well as transparent, in the full meaning of the 

 words; but in extending my experiments to roughened sur- 

 faces, 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 in- 

 creased 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 supe?jicial, 

 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 dif- 

 ference of the following numbers is due to the less degree of 

 roughness of a. 



Source of Heat . 



Rough Salt Plate a 



b 



(a + b) ... 



Per-centage of heat "| 

 received through a > 

 transmitted by b ... J 



Ratio of a to b 



Locatelli, with 

 Glass. 



Per cent. 



30 

 166 



7-2 



24 



100:65 



Locatelli. 



Per cent. 



48-5 

 28-5 

 16 



33 



100:58-5 



Dark Hot Brass. 



Per cent. 



59 

 45 



27-5 



46-5 



100 : 76 



