ON RADIANT HEAT. 65 
partake of the motion of the powerful waves of low refrangi- 
bility, and 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 usiest 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 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 
