RADIANT HEAT AND ITS RELATIONS 95 



experiment miglit be varied in a hundred ways: it proves 

 that from the darkness of a body you can draw no certain 

 conclusion regarding its power of absorption. 



The reason of this simply is, that color gives us intelli- 

 gence of only one portion, and that the smallest one, of 

 the rays impinging on the colored body. Were the rays 

 all luminous, we might with certainty infer from the color 

 of a body its power of absorption; but the great mass of 

 the radiation from our fire, our gas-flame, and even from 

 the sun itself, consists of invisible calorific rays, regarding 

 which color teaches us nothing. A body may be highly 

 transparent to the one class of rays, and highly opaque to 

 the other. Thus the white powder, which has shown itself 

 so powerful an absorber, has been specially selected on ac- 

 count of its extreme perviousness to the visible rays, and 

 its extreme imperviousness to the invisible ones; while the 

 dark powder was chosen on account of its extreme trans- 

 parency 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 invis- 

 ible rays; the body, therefore, which was most opaque to 

 these triumphed as an absorber, though that body was a 

 white one. 



And here it is worth while to consider the manner in 

 which we obtain from natural facts what may be called 

 their intellectual value. Throughout the processes of Na- 

 ture we have interdependence and harmony; and the main 

 value of physics, considered as a mental discipline, con« 

 sists in the tracing out of this interdependence, and the 

 demonstration of this harmony. The outward and visible 

 phenomena are the counters of the intellect; and our sci- 

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



