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226 F RAGMEN TS 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 which the
white powder is strewn, against my cheek or forehead, I
find it intolerably hot. Placing 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

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