110 Professor Forbes's Researches on Heat. 



of polish produce a variation in the proportion of not less than 

 3 to 1 in the quantity of transmitted radiated heat from dif- 

 ferent sources, can we employ salt plates with the ordinary 

 degree of polish, and yet consider them as equally transparent 

 for every kind of heat, as M. Melloni's discovery has hitherto 

 entitled us to do ? (2.) Is the effect of roughness common to 

 other substances as well as rock-salt? (3.) The operation of 

 depolishing with sand-paper is nothing more than the making 

 of an infinite number of distinct grooves on a polished surface; 

 supposing these grooves to be regularly formed, and capable 

 of numerical estimation, will the effect continue? 



27. (1.) With respect to the first of these questions, it is 

 satisfactory to be able to answer it affirmatively in a general 

 way. I took two salt plates, of which the surfaces had not been 

 regularly polished for a long time, and which, though bright 

 and clear, were by no means particularly even and true. Of 

 heat from Locatelli's lamp previously sifted by glass, these 

 four surfaces of rock-salt transmitted 72 per cent. With dark 

 heat from smoked brass the per-centage was 73, a difference 

 which, in this experiment, could hardly be considered as ap- 

 preciable. The transmission of these two very different kinds 

 of heat was therefore equal. M. Melloni has shown that when 

 rock-salt is pure and perfectly polished, *92 of the incident 

 heat is transmitted by a pair of surfaces, and therefore four 

 surfaces should transmit (-92) 2 or 84'5 per cent. This esti- 

 mate I have verified, and am satisfied of its accuracy. The 

 deviation in the present case (which I think it right not to pass 

 over) is due partly, no doubt, to the inequalities of surface, 

 but chiefly to some imperfections in the salt itself, which, as 

 the experiment was merely a relative one, were not adverted 

 to. In contrast with this, I used at the same time (December 

 11, 1839) a piece of salt, which once had been polished on 

 both sides, but which, by being laid aside for some years, had 

 become completely dull and gray on its surface. This speci- 

 men, then, was simply demolished-, it contained no furrows, 

 and had been subjected to no mechanical action whatever. 

 Its per-centage of transmission was, 



Locatelli, with Glass. Dark hot Brass. 



Tarnished salt 66 77 



clearly establishing the general principle. 



28. (2.) With respect to the question, whether roughness 

 of surface has a similar effect in modifying the diathermancy 

 of other substances as well as rock-salt, we are able to give a 

 distinctly affirmative answer. Rock-salt being, so to speak, 

 quite indifferent to the quality and source of the incident heat, 



