vi.] PROFESSORIAL LIFE. 115 



be overwhelmed by ray questions. I have been working 

 lately with Melloni's Thermo-multiplier, and have verified 

 some of his most curious results regarding radiant heat, 

 which in their connection with light are extremely 

 remarkable. It is unfortunate that that point to which 

 his researches chiefly tend is the most obscure in the 

 theory of light, namely, absorption. I have lately 

 succeeded in establishing, as I think for the first time 

 demonstratively and quantitatively, the polarization of 

 non-luminous heat. I abandoned the method of reflec- 

 tion, which is the only one hitherto employed, and 

 adopted that of transmission through piles of thin mica 

 plates, for which the Thermo-multiplier is well adapted ; 

 and with entire success. I have also been endeavouring 

 to determine numerically the refrangibility of non- 

 luminous heat. I discovered, what I now find that 

 M lloni had previously done, that the tourmaline trans- 

 mits almost as much heat when two pieces are placed 

 with their axes crossed as when parallel. Melloni saw 

 quite as much as I also at first found, but I afterwards 

 detected a slight difference/ 



To the KEV. DR. WHEWELL. 



' EDINBURGH, January 1st, 1835. 



'. . . I am <juite full of polarization. I do not exactly 

 understand what you mean by double refraction existing 

 without polarization, but at all events since 1 wrote to 

 Airy I h i\- made great progress. My original experi- 

 ments ivlated io polarization unconnected with double 

 on, but I have sin.-.- . \t< nded the proof to every 

 recognized species of polarization ; and non-luminous heat 

 must be doubly reJVa-i.-d. beeause it can be pnhm/ed by 

 tourmalin.- and d''polari/.'d, or di-pnlari/ed if you will, 

 other crystals. Ilenre \\-e ;nv entitled to conclude 

 t'.-.-l bru-hfs and rays of dark ln-at if our 

 hands were delieah- enough thermometers. I can also 

 make some approximation to the length of a \\a\e. I 

 ' as yet communicated this to no one. I >hall read 

 it on Monday to the Royal Society, when it will be 



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