212 KNOBLAUCH ON RADIANT HEAT. 



condensation of it ; but that no change ensued in this respect 

 when the hardness and elasticity of the surface were not modi- 

 fied by the scratching. 



For the sake of convincing myself of the truth of this inter- 

 esting law, which has not hitherto been further examined, I 

 made the following experiment : — 



I first caused a Leslie's cube of 8 centira., which consisted of 

 two cast and two rolled jilates of lead, and was retained at a tem- 

 perature of 212° by boiling water, to radiate its heat against the 

 thermal pile placed at a constant distance from the heated sur- 

 faces. The surfaces of the two pairs were too different for this 

 experiment to have allowed of our concluding upon the con- 

 nexion of the radiation with the hardness and density. I could 

 not succeed in proportioning the two leaden plates of each pair 

 so as to produce the same deflection of the thermo-multiplier by 

 their radiation. At a certain distance of the heated surfaces 

 from the pile, the radiation of one of the cast plates produced a 

 deflection of the galvanometer-needle to 48°'25, that of the other 

 to 49° ; and the radiation from one rolled plate a deflection of 

 51°, that of the other of 50°-5. 



There was no question that lead becomes condensed at the 

 shining mark made by drawing a steel instrument across it. In 

 conformity with Melloni's conclusion, therefore, the scratching 

 must diminish the radiation of the surfaces of the lead, and 

 this to a greater extent in that cast than rolled. This was 

 confirmed by experiment. When that cast plate which had pro- 

 duced the greater deflection of 49° was scratched, its radiating 

 power was diminished so that it became equal to that of the 

 other surface which radiated less perfectly. They now both pro- 

 duced a deflection of the needle of 48°'25 when at an equal 

 distance from the thermoscope. When the longitudinally 

 scratched leaden plate was covered with transverse stripes, its 

 radiating power became still less. Retaining the same position 

 to the pile, it deflected the galvanometer-needle to 47°'25 only. 



Of the rolled plates, that which produced the deviation of 

 50°*5 was scratched. The radiation was also diminished in this 

 case, for it only caused the needle to deviate to 48°*5, When 

 the surface was scratched in both directions, the radiating power 

 was increased to the extent of producing a deflection of 49°"75 ; 

 which might arise from the lead being condensed as before at 

 the very portions scratched, but rendered less dense at those 



I 



