118 



fore been used, has succeeded in completely establishing the 

 polarization of heat under all the circumstances in which light is 

 polarized, namely, by Reflection, Transmission, and Double 

 Refraction, and that it is for the establishment of these facts 

 that the Keith Prize has been awarded by the Council. 



Dr Hope then stated that, in the ordinai'y case of the publica- 

 tion of papers, the Society holds itself in no degree responsible 

 for the truth of the facts stated therein ; but, in the adjudica- 

 tion of prizes, the case is different ; and that, with regard to them, 

 the Council are bound to be satisfied of the truth of the state- 

 ments for which they award their prize. Several members of the 

 Council had seen and satisfied themselves of the accuracy of Mr 

 Forbes's leading experiments before the Keith Prize was award- 

 ed ; and, some days ago, he deemed it right to request Mr 

 Forbes to shew him the more important of these experimental 

 demonstrations. This he succeeded in doing in a way which 

 left upon his mind not the slightest doubt as to the truth of his 

 results ; the variations of temperature being so obviously dis- 

 played, as to prevent the slightest ambiguity as to the true source 

 from which they are derived. The instrument employed in the 

 research is the thermo-multiplier, of which the invention is due 

 to Nobili, though it has been greatly improved for experimen- 

 tal purposes by Melloni. Professor Forbes has likewise in- 

 creased greatly its power of indicating the more delicate effects 

 by employing a telescopic apparatus, which enables him to mea- 

 sure a quantity of heat, perliaps not exceeding one-Ji/leen 

 hundredth part of a degree of Fahrenheit. 



That the Society may fully understand the nature of the 

 proofs afforded by Mr ^orbes's experiments, reference must be 

 made to the correlative facts observed in the case of light. 



When light undergoes reflection from glass at an angle of SQ", 

 its physical character is found to be thus far altered, that it re- 

 fuses to be a second time reflected by another plate of glass 

 placed to receive the ray at the same angle of 56°, if the plane 

 of incidence on the second glass be perpendicular to the plane of 

 incidence on the first. The light is then wholly transmitted by 

 the second plate. If the plane of incidence be the same for the 



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