Quotation from: General Science

Written by: Bertha M. Clark

[Illustration: FIG. 89.--The energy of the sun can be measured in heat
units.]

Suppose we allow the sun's rays to fall perpendicularly upon a metal
cylinder coated with lampblack and filled with a known quantity of
water (Fig. 89); at the expiration of a few hours the temperature of
the water will be considerably higher. Lampblack is a good absorber of
heat, and it is used as a coating in order that all the light rays
which fall upon the cylinder may be absorbed and none lost by
reflection.