KNOBLAUCH ON RADIANT HEAT. 



199 



however the pile itself indicated elevations and depressions of 

 temperature with the utmost accuracy, which was perfectly suf- 

 ficient for the decision of the present question. 



On transmission by radiation, it was evident that the heat 

 emitted by the metallic cylinder at the increased temperature 

 passed through some substances comparatively better ; through 

 others in the same proportion as on the application of a lower 

 degree of heat. 



Thus the galvanometer needle, which by direct radiation upon 

 the pile was deflected to 35°, receded to 11° on inserting the 

 colourless glass, the cylinder being nine inches from the thermo- 

 scope; but only to 13° when it had so high a temperature as to 

 require removal to a distance of thirty-six inches to produce a 

 similar deviation of 35°. 



With green mica, in the first instance a recession of the 

 needle to 20°*25 was obtained; in the latter to 26°; whilst 

 under all circumstances the needle receded to 3°*5 when the plate 

 of alum 1'4 millim. in thickness was introduced between the 

 source of heat and the thermal pile, and to 8°'5 when gypsum 

 of a similar thickness was used. 



The following table contains the details of the observa- 

 tions : — 



Table III. 



VOL. v. PART XVllI. 



