CHANGE OF LIGHT INTO HEAT 163 



much dust. Atmospheric dust may be ordinary, solid 

 dust, or fine particles (mist) of water or ice (cf. 268). 

 Sunlight dispersed by this dust is " white" daylight (cf. 

 173 and 176). Near the sun and near the horizon the 

 sky is ordinarily light gray. Sunrise and sunset colors 

 are due to the rays that are refracted least (cf. 179) ; 

 hence the indigo and blue of the zenith shade into 

 yellow, orange, and red near the horizon. 



Sky coloring is caused not only by reflection and refraction, but also 

 by an effect which thin clouds of fine particles have in breaking up sun- 

 light into its spectrum. The same effect is produced by a glass plate 

 ruled with many parallel lines, say 15,000 to the inch. The phenome- 

 non is called diffraction. As with the prism, violet rays are bent most 

 and red rays least. 



In 1883 the volcano of Krakatoa, Java, suffered a violent explosion, 

 and so much dust was driven into the upper air that it spread around 

 the whole globe. It produced brilliant sunset effects for 3 years. 



183. Change of Light into Heat. From the study of 

 the spectrum we learned that the violet rays move most 

 rapidly and the red least rapidly. Below the red rays 

 there are rays moving too slowly to give out light of any 

 color, yet having a great deal of energy. These long 

 waves can be changed into heat just as light waves can. 

 Glass will not cut off the light waves, but it will cut off the 

 longer heat waves. We can prove this easily by holding 

 a pane of glass between the hand and a hot stove. Now, 

 when sunlight has been absorbed, and is then radiated 

 again (cf. 66), it no longer consists of the short light 

 waves, but of the long heat waves. The glass of green- 

 houses or hot-beds thus acts as a trap for the sun's energy; 

 for it permits the light waves to pass into the soil; but 



