416 KNOBLAUCH ON RADIANT HEAT. 



tively better through red glass and calcareous spar than that by 

 cinnabar (p. 390 and 391). Therefore if this arose from the car- 

 mine absorbing a larger portion of the rays, which do not pass 

 freely through these bodies, than cinnabar, it must, on compari- 

 son with the latter, reflect the heat of any source less freely the 

 more it emitted to it these rays which are badly transmitted by 

 red glass and calcareous spar. It is moreover known that the 

 heat emitted by the cylinder at a dark red heat permeates red 

 glass and calcareous spar less freely than that of an Argand 

 lamp (p. 402 and 403). Hence carmine, in comparison with cin- 

 nabar, should reflect the heat of the cylinder proportionately less 

 freely than that of the Argand lamp, if this alteration in the heat 

 after reflexion were really produced by selective absorption. Ex- 

 periment has confirmed this; for when the heat of the Argand lamp 

 w^as reflected by cinnabar, the surface being in a certain position, a 

 deviation of 29°'75 in the needle of the multiplier was obtained; 

 whilst the reflexion by carmine, when the reflecting surface was 

 of the same size and in the same position as regards the thermal 

 pile and the source of heat, produced a deflection of 18°'5. How- 

 ever, when the heat of the metallic cylinder, reflected by cinna- 

 bar, had produced a similar deflection of 29°*75, when reflected 

 by carmine, under the same circumstances, it deflected the needle 

 only 14°-37. Thus the intensity of the heat reflected by carmine 

 was really diminished in the manner expected. 



The same was found in the other cases. We know that the 

 heat reflected by white paper is transmitted in a much greater 

 degree by red glass and calcareous spar than that reflected by 

 black paper (see p. 392-394). If this were a consequence of 

 selective absorption, white paper also, in accordance with the 

 above consideration, should reflect the heat of the cylinder at a 

 dark red heat, which transmits principally those rays which are 

 but slightly susceptible of transmission, through red glass and 

 calcareous spar comparatively less freely in comparison with 

 black paper than that of the Argand lamp. 



However, with black paper the contrary ought to occur. In 

 comparison with white paper, it should reflect the heat of the 

 Argand lamp less perfectly than that of the cylinder. This vias 

 found to be the case. The heat of the Argand lamp, reflected by 

 white paper, caused the needle of the galvanometer to deviate to 

 21°"25, whilst that reflected by black paper, under the same cir- 

 cumstances, deflected it to 18°; but the heat of the metallic 

 cylinder, which, when reflected by white paper, caused a devia- 



