Fourth Series. — Hough Surfaces. 



77 



14. It thus appeared that there are at least three conditions 

 under which a medium can be found capable of transmitting 

 heat of low refrangibility, and that two of these had reference 

 solely to mechanical constitution. It was natural to generalize 

 and attempt to include the case of the film of smoke, as well 

 as the striated and the laminated surface, under one category. 

 I have already said that the mechanical distribution of the 

 opake carbonaceous particles offered a plausible analogy, which 

 I proceeded to attempt to carry out. 



15. The numbers in art. 10, may be compared with the 

 following : — 



16. It occurred to me that if the action of the smoke was 

 entirely a superficial one, or due to the character of a rough 

 surface applied to the plate of rock-salt, that the effect of two 

 such surfaces upon the transmission of heat would probably 

 differ from that of a single film of smoke, so thick as to pro- 

 duce an equal absorption of heat of any particular degree of 

 refrangibility. For this purpose I smoked three plates of po- 

 lished rock-salt, so that two marked D and E absorbed toge- 

 ther as much dark heat (very nearly) as the third plate A did 

 alone. 



17. I may take this opportunity of mentioning the way in 

 which I have succeeded in smoking inflammable surfaces 

 without burning them, or crystallized plates, like rock-salt, 

 which crack and fly by the direct application of the flame of 

 a candle. A coarse gas-flame, surrounded by a wide metal 

 tube 10 or 15 inches long, against the side of which the flame 

 partly plays, affords a stream of comparatively cool smoke, 

 which may be applied to any given surface. With these three 

 smoked salt-plates I obtained the following results : — 



Smoked Salt Plate A ... 



D ... 



E ... 



(D + E) 



Source of Heat. 



Locatelli, with 

 Glass. 



Per cent. 



8-3 

 26 

 23-5 



7'3 



Locatelli, 



Per cent. 



172 

 41 

 36 

 18 



Dark Heat. 



Per cent 



32-9 

 58 

 53-5 

 321 



