192 KNOBLAUCH ON RADIANT HEAT. 



The experiments of Delaroche and Melloni, which were made 

 with regard to this point; for directly comparing the transmission 

 of heat from different sources through diathermanous bodies*, 

 appear to indicate that the power of heat to radiate through 

 these bodies increases in proportion to the temperature of its 

 source. 



Thus Delaroche found that a constant number of 10 rays of 

 heat, passing through a glass screen, 



with a source of heat of 357° was contained in 263 rays, 



650° ... 139 ... 



800° ... 75 ... 



1760° ... 34 ... 



in an Argand lamp burning freely . . 29 ... 



and in one furnished with a glass chimney . 18 ... 



And Melloni observed — to give a single example only from 

 among the numerous ones which he has adduced — that 



of 100 rays of heat, which copper emits at 212° . . 33 

 which copper emits at 730° . . 42 

 which red-hot platinum emits . 69 

 from a Locatelli's lamp .... 78 

 pass through a plate of fluor spar 2*6 millim. in thickness. 

 Two observations only foi'm an exception to the position ad- 

 vanced ; for pure rock salt, according to Melloni's investiga- 

 tions, is penetrated by rays of heat from every source in a uni- 

 form manner ; and prepared rock salt, according to Melloni and 

 Forbes, is penetrated by heat in a degree which increases in 

 proportion to the diminution in the temperature of its source. 



Melloni withdraws a former observation, according to which 

 the heat of red-hot platinum passes through black glass better 

 than that of an Argand lamp ; and shows that the instances, be- 

 sides the example adduced, in which Forbes considered that he 

 had observed the better transmission of heat of a lower tempera- 

 ture, had not afforded pure results of radiation, and therefore 

 could not enter into consideration in this point of view. 



The two instances specified therefore stand alone in opposition 

 to an apparently general law. One only directly contradicts it, 

 and relates to a substance which differs from other diathermanous 

 bodies in numerous respects. Hence the supposition of an in- 

 fluence of temperature on the transmission of heat through 



* [On this point the reader should perhaps be referred also to Professor 

 Powell's paper, Phil. Trans. 1825. — Ed.] 



