354 RADIANT HEAT. 



pairs of tourmalines, each employed with different sources of heat) 

 viz: the locatelli lamp, argand lamp, incandescent platina, and copper 

 at 400°. The effect in the latter case was very small. 



In recapitulating his views, the author refers to the unequal absorp- 

 tion of the two pencils in different tourmalines, as causing the 

 differences observed. 



A further paper by the same author, on Tourmaline, &c, in the 

 Ann. cle Chim., April, 1836, displays much ingenuity, but nothing of 

 peculiar novelty or fundamental importance. 



From the Comptes Rendus, 183G, i, 194, it appears that on the 15th 

 February, 1836, M. Arago communicated to the Academy of Sciences 

 a letter from Professor Forbes, announcing his discovery of the circular 

 polarization of heat of the rock-salt rhomb. 



At the next meeting of the same body, '(February 22,) MM. Biot 

 and Melloni stated that in following up Professor Forbes' s experi- 

 ment, they had found that quartz possessed the same "rotative" quality 

 for heat as for light. 



Dr. Thomson, in the second edition of his Treatise on Heat, &c, 

 (1840,) while giving an outline of the discoveries of Forbes and 

 Melloni, has by no means clearly distinguished the share borne by 

 each of those philosophers in the investigation. In particular, with 

 respect to the fact of polarization, he has not given Professor Forbes the 

 credit so unquestionably due to him for the priority of the discovery. 

 He observes, (p. 139,) "In the earlier experiments of Melloni, he did 

 not find that the rays of heat were polarized when passed through the 

 tourmaline. But he afterwards found that this conclusion was hasty, 

 and that the tourmaline polarizes heat as well as light. The truth of 

 this statement is shown very clearly by Professor Forbes. They also 

 polarized heat by plates of mica, and also by reflection," &c. 



These expressions certainly assign the priority to Melloni. as well 

 as an equal share in the subsequent results, both of which we have 

 seen are greatly at variance with the truth. 



Further Researches: Forbes. 



Professor Forbes' s second series of Researches on Heat was read to 

 the Royal Society of Edinburgh, May 2, 1836, and printed both in the 

 Edinburgh Transactions, vol. xiii, and in the London and Edinburgh 

 Journal of Science, vol. xii, 1838. 



The author remarks at the outset, that in his former memoir he had 

 confined himself to the establishment of the general facts of the 

 polarization and ^'polarization of heat, without pretending to accurate 

 quantitative results; he now proceeds, therefore, to a more detailed 

 investigation of the subject, with a view to more precise numerical 

 determinations. 



The first section relates to the methods of observation employed, 

 and the examination of the values of the degrees of the galvanometer, 

 Avhich, for the most part, do not indicate equal increments of force. 

 Two tables are given. By the first, the statical deviations of the 

 needle are reduced so as to be measures of the force producing them; 



