12 REPORT—1840. 
other substance hitherto examined acts on the calorific rays, as 
violet or blue glass does on light, absorbing the rays of least 
refrangibility, and transmitting only the others. And to this 
tule Melloni now makes out the first exception, or the first ana- 
logue of red glass, to be rock salt, having its surface smoked. 
Now Prof. Forbes, in his third series, had also pointed out 
another substance having the same property, viz. mica split by 
heat. In March, 1838, he had established, by repeated experi- 
ments, that the previous transmission of heat through glass, 
far from rendering it less easily absorbable by mica in this 
state, had a contrary effect ; and also that heat of low tempera- 
ture, wholly unaccompanied by light, was transmitted almost as 
freely as that from a lamp previously passed through glass. 
Mica not laminated possesses no such property; hence the 
effect is due to the peculiar mechanical condition of the sub- 
stance: and hence it occurred to the author, that the effect of 
smoking the rock salt was owing merely to a mechanical change 
in the surface ; he therefore proceeded to try the effects of sur- 
faces altered by mechanical means. 
The surface of rock salt being roughened by sand-paper, it 
transmitted non-luminous heat more copiously than luminous. 
Mica similarly scratched showed the same result. 
This effect is not attributable to differences in the proportions 
of heat reflected, for in this respect, at a polished surface, all 
kinds of heat are alike, as he had before shown; whilst by 
direct experiment, he found that, at least for the higher angles 
of incidence, reflexion is most copious from rough surfaces for 
heat of low temperature, or the same kind which is most freely 
transmitted ; proving incontestably, that the stifling action of 
rough surfaces is the true cause of the inequality. 
That there is a real modification of the heat in passing through 
a roughened surface, as well as through laminated mica, and the 
smoky film, appears from some direct experiments on heat sifted 
by these different media; which, when transmitted by any one 
of these, is found in a fitter state to pass through each of the 
others ; and this modification is the more perceptible as the cha- 
racter of the heat is more removed from that which these media 
transmit more readily ; that is, as the temperature of the source 
is higher. The following results were stated :— 
Heat from lamp through Rays out of 100 
smoked rock salt. transmitted. 
Pirectinge 5020s cement tis oo) codes eee 
Previously sifted by another plate of smoked rock salt. 44 
do. do. laminated mica . 44 
do. do. roughened salt . 403 
