KNOBLAUCH ON RADIANT HEAT. 



215 



there is no doubt that the difference between the amount of heat 

 emitted by a polished and a scratched plate of copper in reality 

 diminishes when these surfaces are coated with a uniform layer 

 of this metal. 



The following numbers (each consisting of the arithmetic mean 

 of two observations, afforded by the radiation of differently en- 

 graved plates in the cases which have been considered) will show* 

 this still more clearly : — 



Table IX. 



Kadiating surfaces. 



Plates of 

 rolled copper- 

 plate at 

 212° F. 



Plates of rolled copper, having the same 



inequalities, but coated by galvanism, 



at 212° F. 



Deflection by direct radiation. 



Smooth 



Scratched longitudinally 



Scratched circularly 



Scratched in both directions 



Distance of the plates from the pile in Khenish inches. 

 I 3-25 1 3-25 I 8-00 



33-00 

 34 00 

 35-50 

 35-50 



9-00 



That the differences of the radiation by the galvanic precipi- 

 tate were not perfectly equal is explicable ; for it could not be ex- 

 pected that the copper would be deposited with exact uniformity 

 upon spots of unequal thickness. 



The experiments described have thus confirmed the position ad- 

 vanced by Melloni, that the scratching of the surface influences 

 the radiating property of bodies merely so far as it modifies their 

 density and hardness, and that it increases or diminishes this ac- 

 cording as it loosens or condenses the parts concerned. 



Moreover, the increase which the emission from the metallic 

 surfaces acquires from the copper precipitated by galvanism may 

 also be ascribed to the slight density of this coating in comparison 

 with that of rolled copper. The amount of this is shown by the 

 above table, especially in that example in which the heat emitted 

 by the smooth rolled copper plate produced a deflection of 29° ; 

 that coated by galvanism, at the same temperature and the same 

 distance from the thcrmoscope, a deviation of the needle of 

 49°-25. 



Oxidation of the metal, which as we know also increases the 

 radiation, did not come into play in this phccnomenon, because 

 the copper cube was used for experiment immediately after being 

 coated and whilst the deposit had a bright metallic surface. 



VOL. V. PART XVIII. Q 



