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flection of heat by glass, was pointed out by the Swedish che- 

 mist Scheele. Pictet of Geneva extended his experiments on 

 ihe radiation and the reflection of the heat derived from boilins: 

 water, and our venerable associate Professor Prevost of the same 

 place, established the doctrine of the mobile equilibrium of lieat, 

 in 1802. The triumph of this theory was found in the beauti- 

 ful experiments of Dr Wells, on dew, in 1813. 



Meanwhile, the experiments of Rumford and Leslie were 

 corroborating and extending these general views, even although 

 the doctrines of radiation were denied by the latter philosopher 

 in all his writings. The passage of radiant heat through solid 

 substances, such as glass, and through fluids, such as water, had 

 long been admitted, in the case where light accompanied heat. 

 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 enlargement 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, 

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

 most 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 



