FOURTH SERIES. ROUGH SURFACES. 



equal absorption of heat of any particular degree of refrangibility. For this pur- 

 pose I smoked three plates of polished rock-salt, so that two marked D and E 

 absorbed together 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 suc- 

 ceeded 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, aifords a stream of com- 

 paratively cool smoke, which may be applied to any given surface. With these 

 three smoked salt-plates I obtained the following results : 



As most of these results are from single experiments, the first and the last line 

 must be considered as almost identical, and certainly do not indicate any material 

 specific difference in the absorbent qualities of one thick and two thin films of 

 smoke, which might be expected if the action were a merely superficial one. 



18. From these numbers we deduce another conclusion of some importance. 

 Since a film of smoke transmits most easily heat of low temperature and refran- 

 gibility, we may expect that it will modify the quality of any compound beam of 

 heat which it transmits, and that one such transmission will therefore render a 

 second more easy. Now, we find that the plate D transmitted 26 per cent, of heat 

 from the first of the above sources, and that of the 26 rays escaping from D, and 

 falling upon a second smoked film E, E transmitted 7.3, or 28 per cent, of those in- 

 cident upon it. But by the third line of the table E transmitted 23.5 per cent, only 

 of the direct rays, consequently the capacity for transmission has been increased. 

 In the same way for Locatelli heat we find the per-centage for E raised from 36 

 to 44 by previous transmission through D ; and for dark heat from 53.5 to 56. 



19. Hence a useful application of smoked surfaces to which I have sometimes 

 Jiad recourse. It is often important to operate with more or less refrangible rays 



of heat under exactly the same circumstances of parallelism or divergence, and 

 intensity. Having adjusted an oil-lamp with a salt lens, so as to afford a com- 

 pound beam stronger than required, we may, by interposing a plate of smoked 

 salt, absorb the most refrangible rays, and suffer the others alone to pass, and by 

 then using a glass of proper thickness, the intensity of the heat may be reduced 

 VOL. xv. PAET. i. c 



