KNOBLAUCH OX RADIANT HEAT. 



397 



placed it at 25°. Only heat diffusely reflected by sheet iron 

 passed through the diathermanous substances in the same 

 manner as the unreflected heat, as may be seen from the 

 subjoined table, Avhich contains the details of this investiga- 

 tion : — 



Thus black bodies, as regards luminous rays, observe the same relations as 

 those whicli are coloured to heat. 



The rays of heat reflected by certain homogeneous bodies passed 

 through the diathermanous media in an unaltered proportion. 

 Thus, on inserting the red glass after a direct deflection of 13° 

 had been produced by birch-wood, cork or mahogany, the needle 

 became placed at 8°-08 to 8^-17, and on the insertion of the cal- 

 careous spar, at 18°-50 to 18°-62, by whichever of these three 

 surfaces the heat was reflected, to deflect the needle 25°. 



The same occurred with metals and alloys, in which the pecu- 

 liarity w^as observed, that the heat diffusely reflected by their 

 surface after it had received a " dead" polish, is undistinguish- 

 able on transmission, a result which is in conformity with the 

 proposition laid down by Melloni, that rough metallic surfaces 

 act towards heat in the same manner as white bodies upon light, 

 e. (J. on insertion of the red glass, the needle of the multiplier 

 receded from 13° to 7°-91-7°-75, and on inserting the calcareous 

 spar, from 25° to 15°-33-15°-08, when the rays of heat either 

 directly reached the pile, or were diffusely reflected by gold. 



