00 FRAGMENTS OF SCIENCE. 



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

 the other. Thus the white powder, which lias shown 

 itself so powerful an absorber, has been specially selected 

 on account 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 

 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. 



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 Nature 

 we have interdependence and harmony; and the main 

 value of physics, considered as a mental discipline, consists 

 in the tracing out of this interdependence, and the demon- 

 stration of this harmony. The outward and visible phe- 

 nomena 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 en- 

 deavor, then, to extract from the experiment of Franklin 

 all that it can yield, calling to our aid the knowledge which 

 our predecessors 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 atten- 

 tion on the white piece, let us inquire 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 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 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 fibers and particles are incompetent to 

 absorb the luminous rays. Whether, then, the cloth will 

 sink or not depends entirely upon the dark rays of the sun. 

 Now the substance which absorbs these dark rays with the 



