Proceedings of the British Association. 317 



heat. From this opinion, Prof. P. dissents. The report then 

 proceeds to the second division, on polarized heat, under which 

 a detailed history of the several successive discoveries of Prof. 

 Porbes on this subject was minutely given, with the dates of the 

 several stages of the discoveries. Melloni having failed in repeat- 

 ing the experiments of Berard in polarizing heat by the tourma- 

 lin, and Nobili having in vain attempted it by reflection, Mrs. 

 Somerville, in her Connexion of the Physical Sciences, 2d ed., 

 speaks of it as altogether without experimental proof. In No- 

 vember, 1834, Prof. Forbes took up the subject and obtained com- 

 plete success. He succeeded in polarizing heat from various 

 sources, and by the aid of various substances, as piles of plates of 

 mica, and by reflection and refraction, and showed that the pecu- 

 liar modification of the experiments adopted by Berard, by reflec- 

 tion from glass, the quantity even at the maximum which could 

 reach the thermoscope after two reflections, would be so extremely 

 small, as that no difference of effect in the two rectangular posi- 

 tions could really have been perceptible. The entire series of 

 those discoveries was completed between November, 1834, and 

 January, 1835, the main practical improvement (which led to all 

 the rest of the discoveries.) being the employment of the piles of 

 mica. Prof. Forbes being at Paris in the summer of 1835, and 

 finding both Biot and Melloni sceptical as to these results, he ex- 

 hibited them to those philosophers with mica piles, which he pre- 

 pared for the occasion, and which he left with Melloni. The 

 next subject entered upon by Prof, F. was that of circular and 

 elliptic polarization. This he determined both by depolarization, 

 and also by the internal reflection of heat in a rhomb of rock-salt, 

 as in the analogous cases of circular polarization of light. The 

 report then adverted to the researches of Melloni on polarized 

 heat, and entered minutely into historical details, and the theo- 

 retic views of the author ; and then pointed out some expressions 

 in Dr. Thomson's work on heat, which might lead a person who 

 did not carefully attend to dates and facts, to attribute the priority 

 of discovery to Melloni, and thus to deprive Prof. Forbes of a por- 

 tion of his well-earned fame ;. and which was so clearly his due, 

 that in 1836 the Keith prize had been awarded to him by the 

 Royal Society of Edinburgh, after a close examination by the 

 council of the researches and experiments ; and subsequently the 

 Rumford medal was adjudged to him by the Royal Society of 



Vol. XL, No. 2.— Jan.-March, 1841. 41 



