delivering the Keith Medal. 41 1 



effected in the case of heat, by Mr Forbes. A lamina of mica is in- 

 terposed between the bodies used to polarize heat unconformably 

 placed. When the lamina of mica has a certain position, no effect 

 is produced beyond stopping a small portion of the heat, which 

 would otherwise reach the thermometer; but when this interposed 

 lamina is turned 45° in its own plane, a portion of the heat which 

 before was incapable of reaching the thermometer in consequence 

 of its polarization, is now capable of doing so, and the influx of 

 heat is instantly indicated. The most striking exemplification 

 of this result is found in the fact, which excited so much interest 

 when communicated more than a year ago to the Society, that in 

 certain cases the mere interposition of a piece of mica (in the 

 proper situation), will cause an immediate indication of increased 

 temperature, the mica depolarizing' more heat than it stops. Since 

 depolarization takes place only in consequence of double refrac- 

 tion, we have here another undoubted proof of the double re- 

 fraction of heat. 



The Vice-President terminated his general and rapid sketch, 

 in which he alluded to the brilliant discoveries of Brewster, 

 Arago, and Fresnel, respecting the polarization of light, by ob- 

 serving, that it would be needless for him to point out the im- 

 portant bearing of these facts on the question of the nature of heat, 

 and its connection with light. He concluded in the following 

 terms : — " It now only remains for me to presetit to Professor 

 Forbes the medal which has been awarded to him for these dis- 

 coveries, I believe that I shall be joined cordially by every 

 member of the Society who now hears me, in the fervent wish 

 that it may be the will of the Almighty Ruler, that his life may 

 be long protracted, with vigour of mind and health of body to pur- 

 sue the career in which he has made an advancement so honour- 

 able to himseif, and reflecting lustre upon those great establish- 

 ments, the University and the Royal Society, with which he is 

 connected. I cannot doubt that he will persevere in this happy 

 path with the same ardour and success which have hitherto ac- 

 companied his researches. Indeed, we have a gratifying proof 

 that his zeal will not be impaired, nor his success less brilliant, 

 from the discovery in the same field announced by him at the 

 last meeting of the Society, of the Circular Polarization of 

 Heat." 



