280 BREWER 



violet, the different lengths of the light-waves producing 

 all the colors observed in the rainbow. When the dust- 

 particles in the air are smaller than a wave-length of sun- 

 light, they break up the waves and cause a variety of 

 color effects dependent on the size and other conditions 

 of the particles. The afterglow, seen in the sky just 

 after the sun sets, is caused by particles of dust or ice 

 suspended in the air. The color of the glow is red or 

 orange or yellow, according to the coarseness of the par- 

 ticles, the brilliancy of the coloring being related to cer- 

 tain other conditions. The red color of the disk of the 

 sun or the moon, seen through the smoke of burning for- 

 ests, or through the dense haze in very dry weather, is 

 due to the same cause. So too is the green sun, some- 

 times seen through air laden with finer dust from volcanic 

 eruptions, or even the dust from deserts. 



All the haze and turbidness of the atmosphere and 

 all the colors of the sky are due to suspended particles 

 of dust or smoke or water or ice. Cloud consists of fine 

 particles of water, which become frozen into very fine 

 particles of ice in the cold upper air. The different de- 

 grees of fineness of the suspended particles give different 

 colors to the sky itself. The dust particles are finer in 

 the upper air and they make the sky blue, and the sky is 

 deepest blue where the suspended dust is finest. For this 

 reason the sky is always of a deeper blue when seen from 

 the summits of high mountains. From very high peaks 

 the very dark blue sky has often a violet tinge, due to the 

 exceeding fineness of the dust in the thin, dry, upper air. 



No region has yet been discovered in which the air is 

 entirely free from dust. The shadows of high mountains 

 are often projected against the sky at sunrise and sunset 

 in clear weather and are due to it. The general fact has 

 often been described, particularly as observed from the 

 higher mountains of the western United States, and pho- 



