88 FKAGMENTS OF SCIENCE. 



Which of the cards is then most highly heated? It 

 requires no thermometer to answer this question. Sim- 

 plv pressing the back of the card, on which the white 

 powder is strewn, against the cheek or forehead, 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 sim- 

 ple result abolishes a hundred conclusions which have 

 been hastily drawn from the experiment of Franklin. 

 Again, here are suspended two delicate mercurial ther- 

 mometers 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 re- 

 ceived 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 experi- 

 ment might be varied in a hundred ways: it proves 

 that from the darkness of a body you can draw no cer- 

 tain 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 noth- 

 ing. 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 



