' deliverbig the Keith Medal. 407 



'But in the case of non-luminous heat, it was strenuously denied 

 ♦by Leslie, and others. The experiments of De la Roche proved 

 that such was the fact, at least in the case of heat derived from 

 terrestrial sources, and at the same time luminous. But this sub- 

 ject has received a vast eniargemeut by the recent experiments 

 of Melloni, who has shewn that substances differ surprisingly in 

 •their permeability to heat, and that while some, such as alum, 

 jstop almost every incident ray, others, as rock-salt,^ transmit al- 

 tnost the whole of the heat, and that from whatever source 

 derived. 



The connection of light with heat, was too obvious and import- 

 ant to be overlooked. To Sir W. Herschel the world is in- 

 debted for the first great step in this curious inquiry. He exa- 

 mined the thermometric qualities of the spectrum formed from 

 the sun's rays by a common prism of glass ; and in 1800 an- 

 nounced the curious fact, that the heating power increases, not 

 only from the violet to the red end of the spectrum, but even 

 beyond the latter, indicating the existence of dark calorific rays. 

 These experiments, though at first denied by some authors, were 

 afterwards fully confirmed, and some anomalies which they pre- 

 sented, explained, by Robison, Englefield, Berard, Seebeck, and 

 Melloni. 



Heat, then, even unaccompanied by light, appears to be capa- 

 ble both of reflection and refraction. But new modifications of 

 light, discovered of late years, require us to investigate how far 

 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 disposed, 

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

 Mai us, a most eminent French philosopher and mathemati- 

 cian, discovered the remarkable modification 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- 

 icount of them was published in 1817, in the Memoires (VArcueU. 



Dd2 



