383 



Article X. 



Investigations on Radiant Heat. {Second Memoir.) 

 By H. Knoblauch*. 



[From PoggendorfF's Jnnalen, vol. Ixxi, part 1, April 29, 1847.] 



V. Comparison of the amount of Heat diffusely reflected by 



different bodies. 



It is well known that reflexion occurring in all directions (called 



diffiise) must be distinguished from that which takes place from 



reflecting surfaces at a certain angle only. 



The latter, in reference to heat, has long formed the subject 

 of numerous investigations, which have shown that the intensity 

 of the reflected heat is dependent upon the nature of the re- 

 flecting bodies t, the condition of their surface J, as also the in- 

 clination of the rays incident upon these surfaces §, but that heat 

 from different sources (in all bodies) undergoes this reflexion 

 in the same manner || ^. 



* Translated by Dr. J. W. Griffith. The first Memoir will be found at p. 188 

 of the present volume. 



+ P. V. Musschenbroek, Introd. ad Philos. Natur., 17G2, vol. ii. (De Igne), 

 p. 653. Leslie, An Experimental Inquiry into the Nature and Propagation of 

 Heat, 1804, p. 98. 



X p. V. Musschenbroek, Introd. ad Philos. Natur., vol. ii. p. 654. Leslie, 

 An Experimental Inquiry, &c., p. Q9. 



§ Forbes, Proceedings of the Royal Soc. of Edinb., March 18, 1839. 



y Leslie, I. c. Maycock, Nicholson's Journal, vol. xxvi, p. 75. H. Davy, 

 Elem. of Chem. Phil., vol. i. 



1[ Of course we only allude to true reflexion, which in the ordinary method 

 of proceeding frequently cannot be directly observed. If, with different sources 

 of heat, imperfect diathermanous substances be used for this purpose, e. g. a 

 glass mirror, we should perceive differences on that side in the direction of 

 which the reflexion occurs, accordingly as the incident rays emanate from one 

 or other source of heat. But these differences would not arise from these rays 

 being reflected by the reflecting surface with unequal intensity, but depend 

 solely upon the unequal absorption which they experience on transmission 

 through the glass, before they arrive at the posterior surface, the reflexion of 

 which is in this case observed at the same time. 



Experiments with diathermanous bodies can only lead to accurate results with 

 respect to this point when, as Melloni and Biot have ingeniously done, in the 

 effects observed, the influence of the absorption occurring on transmission is 

 taken into account. They otherwise yield either indefinite results, as the ex- 

 periments of Forbes, Edinb. Trans., vol. xiii., p. 362 ; and of Buff, Wiihler and 

 Liebig's Annal., vol. xxxii. p. ] 70 ; or merely confirm the phaenomena of trans- 

 mission, as e.;i. certain observations of Leslie (An Experimental Inquiry, &c., 

 p. 102-107), in which the amount of heat reflected by a coated metallic mirror 



