8 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



missive power for heat from different sources, and was gratified to find my anti- 

 cipation realized. The proportion of dark-heat transmitted, compared to that 

 from a lamp sifted by glass, was no less than as 3 to 1.* 



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 opaque 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 super- 

 ficial 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 produce an 



* I state it as a proof of the conviction which I had of the real character of split mica with 

 respect to heat, that the reasoning stated in the text was founded upon no experiments made subse- 

 quently to those of March 1838 already quoted. The very first entry in my journal-book of last 

 autumn contains simultaneous experiments, (1.) on smoked salt, to verify M. MELLONI'S observations : 

 (2.) on split mica, to extend my own of March 1838 to perpendicular incidences : (3.) on scratched 

 surfaces, on the assumption that the two former would be realized. As M. MELLONI thinks that I had 

 not a clear idea of the properties of split mica, which, indeed, if I understand him, he still doubts, 

 I will quote verbatim the passage in my laboratory-book alluded to. " 1839, Nov. 12. M. MEL- 

 LONI having lately stated (Comptes Rendus, 2d Sept.) that smoked rock-salt is the only substance 

 known which transmits heat of low temperature easier than luminous, this is in the first place con- 

 tradicted by my experiments of 1838, Mar. 20. &c. on mica split by heat, already published, and 

 in the next place, I felt [feel] some doubt whether [in his experiments] it was the quality of the 

 material or only the surface which affects the result. To try this, and to verify previous experiments, 

 I smoked a plate of rock-salt; I roughened another with sand-paper, first on one, and then on both 

 surfaces ; I had also the split mica plate marked H placed perpendicularly to the rays of heat." 



[Here follow the experiments^ 



" It clearly appears, then, that salt simply roughened transmits most Dark Heat. I presume that 

 the effect of smoking is only superficial, and that roughening stifles luminous heat faster than dark 

 heat." 



This is the first entry in my book after the publication of M. MELLONI'S letter in the Comptes 

 Rendus, and it is given entire. 



