136 Royal Society. 



The Council have awarded the Rumford Medal to Professor 

 Forbes, for his discoveries and investigations of the Polarization 

 and Double Refraction of Heat, published in the recent volumes of 

 the Transactions of the Royal Society of Edinburgh*. 



That solar heat, like the light which it accompanies, may be po- 

 larized, was shown by the early experiments of MM. Malus and 

 Berai-d; but the announcement of M. Berard, that heat from other 

 sources was also capable of polarization, not having been confirmed 

 in subsequent i-epetitions of his exj)eriments by other philosophers, 

 it became of the higliest importance to establish this analogy be- 

 tween light and heat from whatever source the latter might be 

 derived. 



The admirable instrument, the Thernio-multiplier, invented by 

 M]M. Nobili and Melloni, afforded facilities for the prosecution of 

 inquiries of this nature, of which the inventors and others were not 

 tardy in availing themselves. One of the most important results 

 obtained by M. Melloni, and confirmed by Professor Forbes, the 

 refrangibility of non-luminous heat by a prism of rock-salt, appeared 

 to 23oint to the polarization and double refraction of heat as almost 

 necessary consequences. The experiments, however, of both these 

 philosophei's with tourmaline, undertaken nearly at the same time, 

 appeared to negative the fact ; bvit Professor Forbes becoming sen- 

 sible of the source of error, in the conclusions he had at first drawn 

 from his experiments, soon saw that his results clearly indicated the 

 effect he was in search of. His subsequent experiments established 

 the fact, that in the transmission of heat from an Argand lamp, from 

 incandescent platinum, and even from non-luminous heated brass, 

 through slices of tourmaline cut parallel to the axis of the crystal, 

 a portion of the heat is polarized, when the axes of the crystals are 

 at right angles to each other ; and these results were confirmed by 

 M. Melloni. 



But Professor Forbes did not allow the polarization of heat to 

 rest solely upon the results obtained with tourmaline. By employ- 

 ing bundles of plates of mica, and adjusting them at proper angles, 

 he not only obtained much more decisive results, particularly with 

 heat from a non-luminous source, but such results as go to esta- 

 blish the singular fact, that the degree of the polarization of heat is 

 dependent on the nature of its source. He has further shoAvn the 

 depolarization of heat by the interposition of a mica plate, and its 

 circular polarization by means of two total internal reflections in 

 an interposed rhomb, or two prisms of rock-salt. 



The Council consider that they cannot better testify their estima- 

 tion of the discoveries and expeiimental investigations of Professor 

 Forbes, than by awarding to him a Medal, bequeathed by its distin- 

 guished founder, as a premium to the author of discoveries tending 

 to improve the theories of heat and light. 



* See Lond. and Edinb. Phil. Mni;., vol. vi. p. 134 et scq.; vol.xi.p. .542 j 

 vol. xii. p. 545. The papers in which M. J\'Jelloni's Researches are de- 

 tailed have been given in the first volume of the Scientific Memoirs. — 

 Edit. 



