214 KNOBLAUCH ON RADIANT HEAT. 



exposed. Were this the case, the difference in the radiation from 

 polished surfaces and those scratched in both directions should 

 diminish when, without altering their inequalities, these plates 

 are coated with layers of the same metal of equal thickness. 



This may be effected by precipitating copper upon them by 

 galvanism. To procure this coating as uniform as possible by 

 the same electric current upon one polished and thi-ee roughened 

 plates which I wished to examine, I had them so soldered together 

 as to form the lateral walls of a cube, the scratched surfaces being 

 turned inwards. This w^as filled with the cupreous solution, 

 from which the precipitate was formed as usual after the galvanic 

 process was commenced. When it had attained a sufficient 

 thickness, the cube was soldered in such a manner that the 

 surfaces, which were furrowed in both directions and had now 

 become coated, were turned outwards. 



The next question was, whether the difference in their radia- 

 tion was now less than in those plates which were not coated and 

 had the same inequalities. 



This has been experimentally proved most distinctly. Thus, 

 whilst the polished surface of the cube composed of ordinary 

 rolled sheet copper, which before being coated was exactly the 

 same as that already described, at a temperature of 212° F. pro- 

 duced a deflection of 29°, and one of the sides of the same cube 

 which was scratched in both directions a deflection of 47°'75 in 

 the thermo-multiplier, the smooth surface which had been coated 

 by galvanism at the same temperature and distance from the 

 pile, deflected the needle to 49°-25 ; that scratched, to 51°-5. 

 In the first instance the difference amounted to 18°*75 ; in the 

 latter to 2°'25. Certainly the difference between the amount of 

 heat emitted by the smooth and the scratched plate might be 

 equal in both experiments, and yet the difference in the deflec- 

 tions produced by them be less the second time, when observed 

 at higher degrees, than the first time. However, the great dimi- 

 nution in this diffei'ence we have mentioned (from 18°*75 to 

 2°'25), could not be ascribed to this inequality in the indications 

 of the instrument; for when the coated cube was removed so 

 far from the pile that the radiation from its smooth surface de- 

 flected the needle to 33°, the radiation from the scratched plate 

 at the same place produced an indication of 35°"5 in the multi- 

 plier. Thus the difference amounted to only 2°-5, which is less 

 than the deflections comprised in the first observation. Hence 



