DUST. 257 



Tlie dust theory of the splendor of sunset coloring is strength- 

 ened by the often glorious afterglows. The fiercely brilliant 

 streaks of red have disappeared ; over the mountain ridge a flush 

 of orange hovers, and softens the approaching blue. The western 

 hills, that once stood out bronzed against the glare of light, are 

 somber-hued. But suddenly, as by a fairy's wand, the roseate 

 flush of beauty rises in the east, and stretches its beautiful tints 

 all over the sky. As the sun sinks, but before it ceases to shine 

 on our atmosphere, the temperature of the air begins to fall, and 

 its cooling is accompanied by an increase in the size of the 

 particles floating in it by the condensation of the water-vapor. 

 The particles to the east lose the sun first, and are thus first cooled. 

 Accordingly, the rays in that direction are best sifted by the 

 larger water-clad particles of dust, and the roseate coloring is 

 there more distinct than in the north and south. As the sun 

 sinks further, the particles overhead become cooler, and attract 

 the water-vapor ; thus they increase in size, and thereby reflect 

 the red rays. Here the red hues, at first visible in the east, slowly 

 rise, pass overhead, and descend in the west to form the charming 

 afterglow. Sometimes a flood of glory will roll once more along 

 the summits of the hills, entrancing the attention of the artistic 

 spectator. 



All examinations of the volcanic dust lately collected from 

 the atmosphere show that a great quantity of it is composed of 

 small glassy crystals. An abundance of these would quite ac- 

 count for the peculiarity in the visibility of the first glow ; and 

 the evidence seems to indicate that the quantity of such crystals 

 is sufiicient to produce the result. When these are fully illumi- 

 nated, they become in turn a source of illumination, and reflect 

 their reddish light all around. In winter sunsets, the water-clad 

 dust-particles become frozen, and the peculiarly brilliant crimson 

 is seen, coloring the dead beech leaves and red sandstone houses, 

 and making them appear to be painted with vermilion. 



If, then, there were no fine dust-particles in the upper strata 

 of the atmosphere, the sunset effect would be paler ; if there were 

 no large particles in the lower strata., the beautiful sunset effects 

 would cease. In fact, if our atmosphere were perfectly void of 

 dust-particles, the sun's light would simply pass through without 

 being seen, and soon after the sun dipped below the horizon total 

 darkness would ensue. The length of our twilight, therefore, 

 depends on the amount of dust in one form or another in our at- 

 mosphere. Not only, then, would a dustless atmosphere have no 

 clouds, but there would be no charming sunsets, and no thought- 

 inspiring twilights. 



There is a generally prevalent fallacy that the coloring at sun- 

 rise or sunset is much finer when seen from the summit of a 

 TOL. XL. — 21 



