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the analogy may be pursued. In 1802, Dr Young announced 

 his remarkable discovery of the interference of the rays of 

 light, or the power of two luminous rays, properly disposi-il, 

 to produce darkness by their union. About the year 1808, 

 Malus, a most eminent French philosopher and mathemati- 

 cian, discovered the remarkable modificalion which light un- 

 dergoes by reflection from certain substances at certain angles. 

 This modification may be easiest conceived by stating the fact, 

 that light so reflected becomes incapable of undergoing a second 

 reflection in certain positions of the reflecting surface, when com- 

 mon light would be reflected. 



The corresponding experiment in the case of heat was tried 

 by Berard, along with Malus, about the year 1811, and an ac- 

 count of them was published in 1817, in the Memoires d'Arcueil. 

 They found, that when the solar beam was twice reflected in the 

 manner just stated, the heat and light refused simultaneously to 

 be reflected in certain positions of the second reflector. The 

 same experiment was repeated with incandescent bodies, with 

 the same result ; and even, as stated by Berard, with bodies 

 having temperatures beneath that of visible incandescence. 

 These experiments were probably discontinued in consequence 

 of the death of Malus, and the details were never published, if, 

 indeed, they were ever carried to any great extent. The result 

 has been, that Berard's conclusion seems not to have been gene- 

 rally adopted by the scientific world. The polarization of heat 

 has remained amongst the doubtful facts in science. It has 

 been adopted in scarcely any systematic works, whether British 

 or foreign ; and, of late years, direct evidence seemed to be en- 

 tirely against it. Professor Powell of Oxford, repeatedly and 

 fruitlessly, attempted to obtain Berard's result. Nobili of 

 Florence (whose recent loss science has to deplore) attempted it 

 likewise with the aid of his thermo-multiplier, an instrument ad- 

 mirably adapted for the measurement of small quantities of heat ; 

 and Melloni having failed to polarize even luminous heat by 

 tourmalines, concurs in the conclusions of Powell and Nobili. 

 The Vice-President then observed, that it was under these cir- 

 cumstances that the subject was undertaken by Professor Forbes, 

 who, by means of arrangements differing from any that had be- 



