137.  Red a Warm Color.  We have seen that heat and light usually go hand in hand.  In summer we lower the shades and close the blinds in order to keep the house cool, because the 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 heating the water, because if the cylinder is surrounded by blue glass which absorbs the red rays and prevents their passage into the water, the temperature of the water begins to fall.  That the other light rays have a small share would have been clear from the preceding Section.