82 Prof. Magnus on the different Properties of the 



rough surfaces depend upon the fact that in them th« points 

 from which heat is radiated are more numerous ? or is the inten- 

 sity of the individual rays greater? In other words, are the 

 amplitudes of the oscillating particles of rough surfaces greater? 

 Are the rays which are given out by the two kinds of surfaces 

 of the same, or of different heat-colours ? These are questions 

 that have not hitherto been proposed, doubtless because their 

 solution presented difficulties which, for the time being, it was 

 impossible to overcomec 



The observation which I communicated to the Academy of 

 Sciences at Berlin in August last, according to which a platinum 

 wire, heated in the flame of a Bunsen's burner, radiates nearly 

 twice as much heat when it is covered with spongy platinum as 

 it does when the surface is bright, indicated the possibility of 

 answering some of these questions. The following considera- 

 tions served as the starting-point for this investigation. 



Transmission through various Media of the Heat radiated by bright 

 and by roughened Platinum. 



If the rays given out by the two surfaces, the roughened and 

 the bright platinum, were of the same wave-length and of equal 

 intensity, the greater heating-effect produced by the rough sur- 

 face could be caused only by there being in it more particles by 

 which heat was given out. In this case, if the rays were trans- 

 mitted through various media the quantity absorbed would 

 remain always in the same proportion. On the other hand, if 

 the rough surface sent out rays of a different wave-length from 

 those emitted by the bright one, we might assume that among 

 the different media through which the rays were caused to pass, 

 some would be found which would absorb these new rays in greater 

 proportion than the others. The question thus came to be, to 

 examine and compare together the phenomena attending the 

 transmission through various media of the rays emitted by the 

 two kinds of surfaces. 



It was necessary for these experiments that the two surfaces 

 employed should be of exactly the same extent. In order to 

 ensure this, two circular plates of 10 millims. diameter were cut, 

 by means of a hollow punch, from the same piece of platinum- 

 foil, the thickness of which was 0*3 millim. Each of these 

 plates was fastened by three very fine platinum wires to a sup- 

 port, constructed expressly for the purpose, by means of which 

 they could be brought, one after the other, exactly into the same 

 part of the flame of a Bunsen's gas-burner. The flame of this 

 burner, which was supplied with gas through a regulator, was 

 very constant. The platinum disk heated by it was placed 

 parallel to the front surface of the thermo-pile, and so that its 



