88 FRAGMENTS OF SCIENCE. 



requires no thermometer to answer this question. 

 Simply pressing the back of the card, on which the 

 white powder is strewn, against the cheek or fore- 

 head, it is found intolerably hot. Placing the dark 

 card in the same position, it is found cool. The 

 white powder has absorbed far more heat than the 

 dark one. This simple result abolishes a hundred 

 conclusions which have been hastily drawn from 

 the experiment of Franklin. Again, here are sus- 

 pended two delicate mercurial thermometers at the 

 same distance from a gas- flame. The bulb of one of 

 them is covered by a dark substance, the bulb of the 

 other by a white one. Both bulbs have received the 

 radiation from the flame, but the white bulb has 

 absorbed most, and its mercury stands much higher 

 than that of the other thermometer. This experiment 

 might 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 colour gives us 

 intelligence of only one portion, and that the smallest 

 one, of the rays impinging on the coloured body. 

 Were the rays all luminous, we might with certainty 

 infer from the colour 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 colour 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 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 



