90 Prof. Magnus on the different Properties of the 



through alum is still smaller than that which traverses glass ; 

 but, on the other hand, they found that all three substances 

 transmitted the heat-rays of the coloured portion of the spec- 

 trum almost equally well. 



Thus much at any rate results from the experiments that 

 have been mentioned : — 



That the increased radiation which a plate of platinum shows at 

 the same temperature when its surface is roughened does not result 

 from a uniform increase in the intensity of all the rays which it emits, 

 but that the rays whose intensity is increased to the greatest extent 

 lie in the red part of the spectrum and near to it on the dark side. 



Only the amplitudes of the oscillations are altered, not their 

 rate ; for, were this to be changed, new wave-lengths would be 

 produced. But so far as the above-mentioned prismatic investi- 

 gations extend, this does not take place ; for the heat-spectra of 

 both the bright and the platinized platinum have the same extent. 



In consequence of the roughening of the surface, the ampli- 

 tudes of the oscillations are altered, but not the times in which they 

 take place. 



Diminished Intensity of Light with increased radiation of Heat. 



It surprises one that the platinized plate, which gives out 

 nearly twice as much heat as the bright plate, is not more lumi- 

 nous than the latter. On the contrary, the intensity of the light 

 which it radiated seemed always less than that of the light from 

 the bright plate ; and in like manner the spectrum of the pla- 

 tinized platinum was much less luminous than that of the 

 bright platinum. It is true that the mass of the platinum strip 

 is increased by platinizing, and that its bulk likewise is enlarged, 

 so that it might be supposed it would be less heated ; but this 

 cannot be the case ; for when the thickness of the bright strip is 

 increased throughout, or when thicker pieces of platinum are 

 stuck on particular parts, the light which these give is no less 

 than that from the thinner portions. The radiation of heat by 

 a thick piece of the platinum is also the same as that of a thinner 

 piece. If a plate of platinum is platinized on one side only, as 

 was the case in almost all the experiments mentioned above, not 

 only the platinized side, but the bright side also appears darker ; 

 and if platinum-sponge is deposited upon certain parts only of a 

 bright strip, these parts can be detected on the other side as 

 darker patches when the platinum is made incandescent, just as 

 though it were thereby rendered transparent. But bright pla- 

 tinum attached to various parts of a bright strip cannot be de- 

 tected upon the other side. 



If a thin strip of platinum, bright on both sides, is coated on 

 one side with platinum-sponge, the side which remains bright 



