94 FRAGMENTS OF SCIENCE 



of low refrangibility, and consequently cannot be affected 

 by their heat. 



The knowledge we now possess will enable us to ana- 

 lyze 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 di- 

 rect 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 gen- 

 erality 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 

 temperature 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 deli- 

 cate 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 



