78 Professor Forbes's Researches on Heat. 



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 refrangibility, we may ex- 

 pect 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 2G 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 incident 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 in- 

 creased. 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 had recourse. It is often important to 

 operate with more or less refrangible rays of heat under ex- 

 actly the same circumstances of parallelism or divergence, and 

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

 to afford a compound beam stronger than required, we may, 

 by interposing a plate of smoked salt, absorb the most refran- 

 gible 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 in the very same proportion, but the more refran- 

 gible (hottest) rays are alone retained*. 



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 detailed 

 in this paper, incline me to M. Melloni's opinion, that the 



* Smoked glass is evidently an excessively opake compound medium, 

 being composed of two parts which absorb opposite ends of the heat spec- 

 trum. It is curious to reflect how little the true cause of the opacity of a 

 film of smoke deposited upon glass was understood at the time that it was 

 quoted as a convincing proof of the immediate radiation of heat through 

 solid bodies. Far from smoke being the untransparent substance supposed 

 (I use the word loosely in applying it to heat), it transmits a quantity of 

 some kinds of heat really surprising, although the thickness of the smoke be 

 considerable. 



