RADIANT HEAT AND ITS RELATIONS. 227 



class. Thus the white powder, which has shown itself so 

 powerful an absorber, has been specially selected on account 

 of its extreme perviousness to the visible rays, and its ex 

 treme imperviousness to the invisible ones ; while the dark 

 powder was chosen on account of its extreme transparency 

 to the invisible, and its extreme opacity to the visible rays. 

 In the case of the radiation from our fire, about 98 per cent, 

 of the whole emission consists of invisible rays ; the body, 

 therefore, which was most opaque to these triumphed as 

 an absorber, though that body was a white one. _ 



I would here invite you to consider the manner in 

 which we obtain from natural facts what may be called 

 their intellectual value. Throughout the processes of Na 

 ture there is interdependence and harmony, and the main 

 value of our science, considered as a mental discipline, con 

 sists in the tracing of this interdependence and the demon 

 stration of this harmon}\ The outward and visible phe 

 nomena are with us the counters of the intellect ; and our 

 science would not be worthy of its name and fame if it 

 halted at facts, however practically useful, and neglected 

 the laws which accompany and rule phenomena. Let us 

 endeavor, then, to extract from the experiment of Franklin 

 its full intellectual value, calling to our aid the knowledge 

 which our predecessors have already stored. Let us im 

 agine two pieces of cloth of the same texture, the one 

 black and the other white, placed upon sunned snow. Fix 

 ing our attention on the white piece, let us inquire whether 

 there is any reason to expect that it will sink into the 

 snow at all. There is knowledge at hand which enables 

 us to reply at once in the negative. There is, on the con 

 trary, reason to expect that after a sufficient exposure the 

 bit of cloth will be found on an eminence instead of in a 

 hollow ; that instead of a depression, we shall have a rela 

 tive elevation of the bit of cloth. For, as regards the lu 

 minous rays of the sun, the cloth and the snow are alike 



