RADIANT HEAT. 353 



conditions of the last case. The effect increases with the obliquity 

 of incidence. 



All these results have been verified in the case of obscure as well 

 as luminous sources of heat. 



On the 15th Feb., 1836, the Keith prize was awarded to Professor 

 Forbes by the Royal Society of Edinburgh; the Vice President, Dr. 

 Hope, stating-, in the course of a most able address delivered on the 

 occasion, that several members of the council, as well as himself, had 

 personally witnessed the satisfactory verification of the main facts an- 

 nounced before the medal was adjudged. 



Polarization of Heat from different sources: Melloni. 



M. Melloni' s first memoir " on the Polarization of Heat," was read 

 to the Academy of Sciences in January, 1836; it appears in the Ann. 

 de Ghim., lxi, April, 1836, and is translated in Taylor's Scientific 

 Memoirs, Part II, p. 325. 



The author commences with a fair review of the previous investi- 

 gations on the subject, admitting Professor Forbes' s discovery,, but 

 remarking the very small amount of the effect in the case of obscure 

 heat. 



He adopts the supposition that "the different temperatures of the 

 calorific rays are to radiant heat what the different colors of the lumin- 

 ous rays are to light." The latter, he observes, are all equally 

 polarizable, and thus he is led to regard the difference of polarizability 

 in the rays of heat as rather apparent than real. His object then, in 

 this memoir, is to examine the question of the reality of the polariza- 

 tion of heat, and of the equality of the effect in different sorts of heat. 



After some considerations on the general natur% of the apparatus to 

 be employed, and overcoming the difficulty arising from the small 

 total intensity of the rays, by concentrating them by means of a rock- 

 salt lens, he proceeds to detail his several series of experiments, the 

 results of which he gives in the form of tables : 



Table I gives the' different indices of polarization obtained with 

 nine sorts of tourmalines of different color, the source of heat being 

 a locatelli lamp. 



He then tried the experiment, taking that pair of tourmalines which 

 gave the greatest effect in the last set, with plates of various sub- 

 stances interposed between the lamp and the apparatus. Of these, 

 opaque black glass rendered the effect nearly insensible, other solids 

 and liquids of various degrees of transparency produced effects of 

 different magnitude. 



In Table II these results are registered, and the properties of the 

 media, in this respect, were found to follow the same proportion as 

 their diathermancy. 



The author considers the difference of the tourmalines in this 

 respect as referrible to the same cause. 



Table III gives similar results with another pair of tourmalines, in 

 which case the proportions are found to differ. 



In Table IV are given the indices of polarization with four different 



