RADIANT HEAT AND ITS RELATIONS. 89 



to the visible, rays. In the case of the radiation from 

 our fire, about 98 per cent, of the whole emission con- 

 sists of invisible 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 pro- 

 cesses of Nature we have interdependence and harmony; 

 and the main value of physics, considered as a mental 

 discipline, consists in the tracing out of this interde- 

 pendence, and the demonstration of this harmony. The 

 outward and visible phenomena are 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 the phenomena. Let us endeavour, then, to ex- 

 tract from the experiment of Franklin all that it can 

 yield, calling to our aid the knowledge which our pred- 

 ecessors have already stored. Let us imagine two pieces 

 of cloth of the same texture, the one black and the 

 other white, placed upon sunned snow. Fixing our 

 attention on the white piece, let us enquire whether 

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

 snow at all. There is knowledge at hand which enables 

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

 contrary, reason to expect that, after a sufficient ex- 

 posure, the bit of cloth will be found on an eminence 

 instead of in a hollow; that instead of a depression, 

 we shall have a relative elevation of the bit of cloth. 

 For, as regards the luminous rays of the sun, the cloth 

 and the snow are alike powerless; the one cannot be 

 warmed, nor the other melted, by such rays. The 

 cloth is white and the snow is white, because their 

 confusedly mingled fibres and particles are incompetent 



