350 RADIANT HEAT. 



his own theory, which is grounded essentially on the assumption that 

 the radiation of heat from substances at the bottom goes on through the 

 water ; and partly, also, on the supposed greater radiation from dark- 

 colored surfaces. Neither of these assumptions, it appears to me, are 

 admissible ; the former, especially, is 'directly at variance with the 

 experiments of Melloni. Some suggestions, at least, towards a theory 

 not open to these objections are given by an anonymous writer in the 

 Magazine of Popular Science, vol. i, p. 157. 



DIVISION II. — POLARIZED HEAT. 



Polarization of Heat : Forbes. 



The original statement by Berard, of the polarization of heat by 

 reflection, and the attempts to verify it, are mentioned in my former 

 report.* In 1833 Melloni tried to repeat the experiment with tour- 

 malines, but unsuccessfully, t 



In 1834 Nobili attempted it by reflection, employing the thermo- 

 multiplier, but without success. | The disbelief in such a result, at 

 least with dark heat, seems now to have prevailed generally. Mrs. 

 Somerville, in the second edition of her " Connexion of the Sciences," 

 (in 1833,) speaks of it as altogether without experimental proof. 



Professor Forbes took up the inquiry in November, 1834; and in 

 his first memoir, already referred to in section 2, announced his com- 

 plete success, after having in the first instance failed from the influ- 

 ence of secondary radiation, which disguised the real effect. 



(1.) He proved distinctly the stoppage of a considerable proportion 

 of heat when the tourmalines were crossed, not only with a lamp, 

 but with brass heated below luminosity. 



(2.) In the third section of the same memoir he details his researches 

 on the polarization of heat by refraction and reflection. In the former 

 he employed piles of mica, and through these found even dark heat 

 very freely transmitted at the polarizing angle. Without (in this 

 stage of the inquiry) aiming at quantitative results, he found in 

 general that the proportion of heat polarized varied with the source 

 in the following order, beginning with the highest: 



Argand lamp. 



Locatelli lamp. 



Spirit lamp. 



Incandescent platina. 



Hot brass, about 700° Fahrenheit. 



Mercury, 500° in crucible. 



Water under 200°. 

 (3.) The polarization of heat by reflection at the surface of a pile 

 of plates of mica was also established; and with regard to the reflection 

 from glass, Professor Forbes has also remarked that, from the known 



s Pages 301, 312. 



f Second Memoir, Ann. de Chim,, 55. Taylor's Scientific Memoirs, Part I, p. 59. 



j BtbW'th. Univ., September, 1834. 



