^{page 4 note *} Essay on Heat, 1804.

^{page 4 note †} Phil. Trans. 1825, p. 187.

^{page 4 note ‡} Ann. de Chimie, Avril 1834. M. Melloni finds, for instance, that the rays from an oil-lamp falling on black and white surfaces, affects their temperature in the proportion of 1000: 805. And the same proportion holds if they be transmitted through a plate of rock-salt; but if a plate of alum be used, though equally transparent for light with the salt, the proportion is now 1000: 429.

^{page 5 note *} Annales de Chimie, Avril 1834.

^{page 5 note †} Researches on Heat, Third Series, art. 73, 81, &c.

^{page 8 note *} 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 subsequently 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. Melloni 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 contradicted 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.”

[Herefollow 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.

^{page 10 note *} Smoked glass is evidently an excessively opaque compound medium, being composed of two parts which absorb opposite ends of the heat spectrum. 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.

^{page 11 note *} See Melloni, Ann. de Chimie, Dec. 1835, and my Memorandum on the Intensity of Reflected Heat and Light, Proceeding's Royal Society of Edinburgh, p. 254.

^{page 12 note *} Edinburgh Transactions, vol. xiv. p. 371.

^{page 17 note *} Yet an alum plate of a certain thickness transmits no less than 27 per cent. of the one kind of heat, and no sensible portion of the other (Melloni).

^{page 17 note †} See note page 1 of this paper.

^{page 23 note *} To put this is in the most clear point of view, I used and compared two such plates in the same experiment.