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Yj^. Researches on Heat. Third Series. ^ \. On the unequally PolarizaUe Nature 

 of different kinds of Heat. § 2. On the Depolarization of Heat. § 3. On the 

 Refrangibility of Heat. By James D. Forbes, Esq. F. R. SS. L. Sf E., Pro- 

 fessor of Natural Philosophy in the University of Edinburgh. 



Read 16th April 1838. 



Introductory. 



1. The following paper is divided into three sections, containing three distinct 

 yet intimately connected investigations. The two first on the Polarizabihty and 

 Depolarization of Heat have arisen immediately out of the train of investigation 

 contained in my two former papers, and the researches of others to which they 

 gave rise. The third is on the Refrangibility of Heat, a point of the highest im- 

 portance for theory. 



2. The experiments on which these investigations are based have been per- 

 formed almost exclusively dming the past winter. Part of the experiments on 

 Depolarization were, however, made in the winter 1836-7. The mode adopted for 

 trying Refractive Indices I had long ago contemplated. It was not, however, put 

 in practice until January last. 



3. The methods of measuring heat, &c. are exactly those fuUy detailed in the 

 Second Series, § 1. The only modification of importance was attaching a lens in 

 front of the pile, as described in Art. 56 of this paper. 



4. During the two years which have elapsed since the publication of the 

 Second Series, I have not discovered any correction which I have to make upon 

 the statements of my former papers, excepting as to the measure of the polarizing 

 power of a pile of plates of rock-salt (Second Series, art. 25), which I find to be 

 inexact. 



^ 1. On the unequally PolarizaUe Nature of different kinds of Heat. 



5. It has been my anxious wish to preserve these papers pure from even the 

 appearance of controversy, and such members of the Society as have paid attention 

 to the recent history of our present subject must be aware that, without making 

 direct allusion to the doubts Avhich have at different times been thrown upon my 

 experiments, I have contented myself with adducing new facts and more convin- 

 cing reasonings ; and I have had the satisfaction to see that the general result 



