KNOBLAUCH ON RADIANT HEAT. 



231 



Table XIX. {continued). 



Thus the heat emitted by adiathermanous and diuthermanous 

 bodies, within the limits of these experiments, passes through 

 media used for testing them in exactly the same manner. Hence 

 (as is also evident from the values given) it is a matter of indif- 

 ference ivhether the radiating substances [e. g. black lac or white 

 glass) are of a greater or less thickness. 



I have yet to detail my reasons for considering as diatherma- 

 nous the bodies last examined, in which, when acted upon by 

 the Argand lamp, I found differences in the transmission 

 through red and blue glass, alum, rock salt, calcareous spar and 

 gypsum (see Table XVII.). 



No one will hesitate to admit that heat directly penetrates 

 colourless glass and the pores of silk-cloth ; but it might be a 

 matter of doubt whether a transmission in its true sense, although 

 only diffuse, could be admitted to occur in pei'fectly opake glass 

 and lac, or in a layer of carmine, paper and ivory. That this is 

 really the case, the following investigation will prove. 



If any body, e-x;cept soot and metal (see p. 188, 189 and 

 20G), be placed successively before the blackened theimoscope 

 to the rays of different sources of heat, e.g. an Argand lamp 

 and a metallic cylinder heated to 212°, which directly exert 

 an equal action upon the instrument, it indicates different 

 degrees. This difference in the indications may either arise 

 from the substance inserted being adiathermanous, and becoming 

 unequally heated when under the influence of different sources 

 of heat, or that it is diatherra.-inous and allows of the passage of 

 various kinds of rays to an unequal extent ; or, lastly, that the 

 effects observed upon the thermoscope are partly produced by 

 the screens becoming heated, partly by the rays which pass 

 through them. 



VOL. V. PART XVIir. H 



