182 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



21. When I had fully seized this conclusion, the explanation of M. Mklloni's 

 results was easy and complete. It appears from the account of his experiments, 

 that he still employs piles of mica of the form I at first used, consisting of distinct 

 laminge separated by a knife, then laid together and united at the edges, up to 

 the number of 30, 60, and even more.* On the other hand, the piles I and K, 

 which for two years and a half I have employed, are of a degree of tenuity reaUy 

 surprising. The mode of their construction I mentioned briefly in my last paper, 

 art. 20, and it is so very superior to any other, that it is probably from inadver- 

 tence that it has not been generall}^ employed. The piles laminated by the action 

 of violent heat, afford a multiplicity of parallel surfaces in a given thickness of 

 mica, which no mechanical method can approach. The actual thickness of mica 

 which they contain, I am unable accurately to estimate. The plates marked G 

 and H are much thicker, perhaps twice as thick as those marked I and K, which 

 I commonly use ; yet the former, as I roughly estimate by the tint they give in 

 polarized light, are only about one-thousandth of an inch in thickness. At the 

 utmost, the plates I and K can be but one fifteen-hundredth of an inch ; and yet it 

 appears that their polarizing power (depending solely on the number of surfaces 

 they contain) is equal to M. Melloni's pile of ten distinct plates placed at the same 

 angle (35° to the incident rays). The mean thickness of the elementary plates 

 can, therefore, be only one fifteen-thousandth of an inch ; and they reflect abun- 

 dantly the colours of Newton's rings. 



22. Now, I have found by the depolarization experiments, that it requires a 

 much greater thickness of mica than that traversed by the heat in passing through 

 the plates I and K (even allowing for the obliquity) to affect materially the index 

 of polarization of heat from different sources, such as from brass at 700°, and in- 

 candescent platinum. It is, therefore, a necessary consequence of the construc- 

 tion, that the heat passes through such pUes as I use unaltered, or nearly unal- 

 tered, in its character, whilst in passing through bundles of detached plates laid 

 together, the thickness of mica to be traversed is sufficient to modify the heat by 

 absorption, in such a way that the difference of quality has vanished, whatever he 

 the source, in the very act of t7'ansmission. It is hardly likely, considering the size 



* Annates de Chimie, Mai 1837. At p. 17, &c., M. Melloni has given a minute account of that 

 method of constructing the piles, which, " amongst several different ways, he considers the preferable 

 one." No one could doubt from his language that he is describing a new and improved form of the 

 apparatus. I regret for a moment to descend to notice an apparent want of justice and courtesy to- 

 wards myself; but it is impossible for me not to observe, that the procedure he so exactly details, is, 

 to almost the minutest particular, identical with that which I myself used in June 1835, in constructing, 

 in M. Melloni's presence, the first pair of piles used for polarizing heat which existed in France, at a 

 time when M. Melloni expressed his unqualified scepticism as to the polarization of heat generally ; 

 which piles I left, at his desire, where I presume they now are, — in his own possession. This mode of 

 construction I soon after abandoned, for the improved one alluded to in the text. 



