he exclusion of light means the
exclusion of some heat; in winter we open the blinds and raise the
shades in order that the sun may stream into the room and flood it
with light and warmth. The heat of the sun and the light of the sun
seem boon companions.

We can show that when light passes through a prism and is refracted,
forming a spectrum, as in Section 127, it is accompanied by heat. If
we hold a sensitive thermometer in the violet end of the spectrum so
that the violet rays fall upon the bulb, the reading of the mercury
will be practically the same as when the thermometer is held in any
dark part of the room; if, however, the thermometer is slowly moved
toward the red end of the spectrum, a change occurs and the mercury
rises slowly but steadily, showing that heat rays are present at the
red end of the spectrum. This agrees with the popular notion, formed
independently of science, which calls the reds the warm colors. Every
one of us associates red with warmth; in the summer red is rarely
worn, it looks hot; but in winter red is one of the most pleasing
colors because of the sense of warmth and cheer it brings.

_All light rays are accompanied by a small amount of heat, but the red
rays carry the most._

What seems perhaps the most unexpected thing, is that the temperature,
as indicated by a sensitive thermometer, continues to rise if the
thermometer is moved just beyond the red light of the spectrum. There
actually seems to be more heat beyond the red than in the red, but if
the thermometer is moved too far away, the temperature again falls.
Later we shall see what this means.

138. The Energy of the Sun. It is difficult to tell how much of the
energy of the sun is light and how much is heat, but it is easy to
determine the combined effect of heat and light.

[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.

Light and heat rays fall upon the lampblack, pass through the
cylinder, and heat the water. We know that the red light rays have the
largest share toward heat