316 Proceedings of the British Association. 



The report then went on to the researches of Melloni on the 

 reflection of heat ; and the analogies of hght and heal, as traced 

 by Forbes and Melloni. He dissents from the opinion of Am- 

 pere, that the difference between heat and light is to be accounted 

 for by the difference of wave length on the undulatory hypoth- 

 esis. Daring these researches, he found that a certain kind of 

 green glass colored by oxide of copper, though it permitted a por- 

 tion of luminous rays to pass, absorbed all the calorific rays, so 

 that it exhibited no calorific action capable of being rendered per- 

 ceptible by the most delicate thermoscope, even when so concen- 

 trated by lenses, as to rival the direct rays of the sun in brilliancy. 

 With respect to the transmission of heat by screens. Prof. F. re- 

 marked that Melloni's view of the transmission of heat of low 

 temperature, by all substances alike, is equivalent to saying that 

 substances in general allow only the more refrangible rays to 

 pass, or that while rock-salt presents the analogy of white glass, 

 by transmitting all rays in equal proportions, every substance 

 hitherto examined acted on the calorific rays as violet or blue 

 glass does on light, absorbing the rays of least refrangibility, and 

 transmitting the others only. To this rule, Melloni made out 

 the first exception, or the first analogue to red glass, — rock-salt 

 with its surface smoked. Prof. F, soon pointed out another, viz, 

 mica split by heat into numerous fine laminse, and hence, as the 

 effect was obviously mechanical, (since unlaminated mica pro- 

 duces no such effect,) he concludes that the smoked surface of 

 the rock-salt acted also mechanically, and was thus led to try the 

 effects of surfaces variously altered by mechanical means ; and 

 thus effects in some distant degree analogous to sifting the heat, 

 were observed. Fine powders, also sifted on the surface, were 

 found to affect the transmission of heat, and these Prof. F. con- 

 sidered analogous to diffraction and periodic colors in light. From 

 these important researches, we have learned to connect modifica- 

 tions in the transmission of heat with the quality of refrangibilitjr, 

 and not as heretofore with a supposed difference of quality de- 

 pending on the source of the heat. The report then gave an ac- 

 count of the researches of Dr. Hudson on radiation of heat, those 

 of President Bache and Stark on the influence of color and sur- 

 face on radiation, and Prof. P.'s experiments upon the repulsive 

 power of heat ; and adverted to Mr. Farquharson's theory of the 

 formation of ice at the bottom of rivers, as a result from radiant 



