KNOBLAUCH ON RADIANT HEAT. 42/ 



If the result previously obtained (p. 233 and 234), which showed 

 that the heat radiated from the most different solid bodies be- 

 tween 88° and 234° F. is homogeneous, be simultaneously borne in 

 mind, it is evident that within these temperatures the rays of heat 

 emitted by them all — to make use of an expression which reminds 

 us of Melloni's terminology — are of one and the same "colour." 



We have thus advanced to a certain limit, at which every 

 variation of the rays of heat vanishes, a limit which is not at- 

 tained until long after the differences in the luminous rays have 

 become invisible. 



It appeared to me of interest to ascertain. How the heteroge- 

 neity in the rays of heat emitted by one and the same body is 

 affected by its temperature. 



In investigating this, I had the two means just described at 

 my command, i. e. the heat of the heated body, in those stages in 

 which I wished to ascertain its compound nature, might either be 

 reflected diffusely by different surfaces, or be transmitted through 

 different bodies, before passing through certain diathermanous 

 substances. In both cases, as we have seen, differences occur, 

 which appear to be greater and more varied the more different 

 the kinds of rays are which emanate from the source of heat. 



I preferred the first process, because it was possible to pre- 

 vent, by means of cold water, the disturbing influence resulting 

 from the reflecting surfaces themselves becoming heated (see 

 p. 385-387) ; whilst in the second case the body first radiated 

 through could not be prevented from becoming heated, and its 

 influence upon the experiment could only be eliminated at the 

 expense of the intensity of the effects. 



As in the former experiments (p. 201 and 202), to decide the 

 present question, I also heated a spiral of platinum over the chim- 

 ney of a Berzelius's lamp, first at a temperature below 234° F., 

 then at a red, yellow and white heat. 



The heat of platinum below 234° F., in correspondence with 



stantly caused a recess of the needle from 36° to 10°-1, when immediately 

 transmitted to the thermal pile from the metallic cylinder, or after having per- 

 meated post paper or white glass (Table XVIII.) ; at a certain higher tempe- 

 rature, after the same direct deflection of 35°, on inserting the red glass it pro- 

 duced a deflection of lO°-75 when emanating immediately from the cylinder, 

 of 11° 75 after having permeated the paper, and of I4°*75 when it had passed 

 through the white glass. 



The same occurred with the other diathermanous bodies. It might be ima- 

 gined that this variation in the heat emitted at higlier temperatures was de- 

 pendent upoji the alteratiou of the capacity for heat with the increase of tem- 

 perature. 



