ON RA DIA N T UK. I / . 65 



partake of the motion of the powerful waves of low refrangi- 

 bility, 'iiid consequently cannot be affected by their heat. 



The knowledge we now possess will enable us to analyze 

 with profit a practical question. White dresses are worn 

 in summer, because they are found to be cooler than dark 

 ones. The celebrated Benjamin Franklin placed bits of 

 cloth of various colors upon snow, exposed them to direct 

 sunshine, and found that they sank to different depths in 

 the snow. The black cloth sank deepest, the white did 

 not sink at all. Franklin inferred from this 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. One of these two cards is coated with 

 a very dark powder, and the other with a perfectly white 

 one. I place the powdered surfaces before a fire, and 

 leave them there until they have acquired as high a tem- 

 perature as they can attain in this position. Which of the 

 cards is then most highly heated?- It requires no ther- 

 mometer to answer this question. Simply 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 simple result abolishes a hundred con- 

 clusions which have been hastily drawn from the experi- 

 ment 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 sub- 

 stance, 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 experi- 

 ment might be varied in a hundred ways: it proves that 

 from the darkness of a body you can draw no certain con- 

 clusion 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 tho rays all 

 luminous, we might with certainty infer from the color of 

 a body its power of absorption; but the great muss 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 



