226 FRAGMENTS OF SCIENCE. 



sank the deepest, the white did not sink at all. Franklin 

 inferred from his experiment that black bodies are the best 

 absorbers, and white ones the worst absorbers, of radiant 

 heat. Let us test the generality of this conclusion. I have 

 here two cards, one of which is coated with a very dark 

 powder, and the other with a perfectly white one. I place 

 the powdered surfaces before the fire, and leave them there 

 until they have acquired as high a temperature as they can 

 attain in this position. Which of the cards is most highly* 

 heated ? It requires no thermometer to answer this ques- 

 tion ? Simply pressing the back of the card, on w r hich the 

 white powder is strewn, against my cheek or forehead, I 

 find it intolerably hot. Placipg the dark card in the same 

 position I find it 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 suspended 

 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. I might vary 

 this experiment in a hundred ways, and show you 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 trans- 

 parent to one class of rays, and highly opaque to the other 



