218 KNOBLAUCH OX RADIANT HEAT. 



cording as it is heated to a given degree ly rays from different 

 sources of heat ? 



To decide this point, I coated thin paper, stretched upon a 

 metallic frame, on both sides with lamp-black so thickly that 

 direct transmission of the heat was impossible. When this was 

 exposed immediately before the thermal pile to the rays of an 

 Argand lamp or a metalHc cylinder at 212' F., which produced 

 the same direct deflection of the thermoscope, certain deflections 

 of the multiplier were obtained, which were occasioned by the 

 laj'ers of lamp-black becoming heated on one side and their 

 radiation upon the other. The effect of absorption was found in 

 former experiments (pp. 188 and 189) to be the same in both 

 cases ; hence if these deflections corresponded with each other, 

 the radiating power must also be the same in both cases. This 

 was really the case. The needle deviated to 9"'87, whether the 

 rays of the Argand lamp or of the heated cylinder acted upon 

 the blackened surface, provided that each of these sources of heat 

 had produced the same direct deflection of 35° in the pile. 



The same I'esult w"as obtained when carmine or black paper 

 Avas caused to radiate upon the thermoscope ; for when the 

 former, with the blackened side next the source of heat, was ex- 

 posed to the pile, an indication of 10°'5 was obtained in the mul- 

 tiplier, whether the heat was imparted by the flame of the 

 Argand lamp or the dark source of heat; and when these sur- 

 faces were replaced by black paper, which was also coated with 

 lamp-black on the side next the heating rays, each time a devia- 

 tion of 10°, or nearly so, was observed in the needle when the 

 direct deflection amounted to 35°. 



It is thus shoivn that the radiating j^ower of the bodies exa- 

 mined is the same, be the calorific rays by which they are heated 

 of ever so different kinds. 



The subjoined table contains the details of the observations 

 (arithmetic means of every two numbers) : — 



i 



