CONDUCTION AND RADIATION. 547 



mained doubtful. But soon after this time, MM. 

 Melloni and Nobili invented an apparatus, depend- 

 ing on certain galvanic laws, of which we shall 

 have to speak hereafter, which they called a iher- 

 momultiplier ; and which was much more sensitive 

 to changes of temperature than any previously- 

 known instrument. Yet even with this instrument, 

 M. Melloni failed, and did not at first, detect any 

 perceptible polarization of heat by the tourmaline 27 ; 

 nor did M. Nobili as , in repeating M. Berard's ex- 

 periment. But in this experiment the attempt was 

 made to polarize heat by reflection from glass, as 

 light is polarized : and the quantity reflected is so 

 small that the inevitable errours might completely 

 disguise the whole difference in the two opposite 

 positions. When Prof. Forbes, of Edinburgh, (in 

 1834) employed mica in the like experiments, he 

 found a very decided polarizing effect; first, when 

 the heat was transmitted through several films of 

 mica at a certain angle, and afterwards, when it was 

 reflected from them. In this case, he found that with 

 non-luminous heat, and even with the heat of water 

 below the boiling point, the difference of the heating 

 power in the two positions of opposite polarity (pa- 

 rallel and crossed) was manifest. He also detected 

 by careful experiments 29 , the polarizing effect of 

 tourmaline. This important discovery was soon con- 



27 Ann. de Chimie, vol. Iv. M Bibliotheque Universelle. 



29 Ed. R. S. Transactions, vol. xiv. ; and Phil. Mag 1835, 

 vol. vi. p. 209. Ib. vol. vii. p. 349. 



NN 2 



