THE WORK OF THE ATMOSPHERE 19 



Explosive volcanoes often send great quantities of rock matter, 

 broken up into fine particles, high into the air. This is volcanic dust. 

 It is also called volcanic "ash," but this name is not a good one, 

 because the dust is not the product of burning. It is lava blown 

 into tiny bits. 



In August, 1883, a violent volcanic eruption took place on the 

 Island of Krakatoa, between Java and Sumatra, and half the island 

 was blown away. At the same time, enormous quantities of dust 

 were shot up into the air to the height of several miles, and some of 

 it was blown great distances before it fell. Dust in the air affects 

 the sunlight coming through it, and when the sun is low, as at sun- 

 set, the dust effects give color to the western sky. After the erup- 

 tion, bright sunsets occurred at first near Krakatoa, and then 

 farther and farther away. In this way it was estimated that the 

 volcanic dust was carried completely around the earth, and that it 

 took only about fifteen days to make the circuit. It is probable 

 that the dust from this single volcanic eruption found its way to 

 nearly all parts of the earth. 



Some dust reaches the earth from space outside the earth. 

 " Shooting stars" are small bits of solid matter which enter the 

 atmosphere from outside space. In falling toward the earth, they 

 become glowing hot, because of friction with the air through which 

 they pass. Before they reach the bottom of the air, they are broken 

 into tiny particles of dust. This dust is sometimes called cosmic, 

 or meteoric dust. 



How held in the air. Particles of dust are very much heavier 

 than the air, and gravity tends to bring them down. In spite of 

 this, they remain in the air (1) because they are so small that they 

 do not fall readily, and (2) because there are many upward currents 

 in the air which carry the particles upward in spite of gravity. As 

 a matter of fact, the dust of the atmosphere is always settling, and 

 the supply is being renewed constantly. 



Deposits of dust. From the flood plains of such rivers as the 

 Missouri, clouds of dust are swept up and out over the neighboring 

 uplands, whenever the surface of the flood plain is dry and the wind 

 strong. The deposits of dust on the uplands in any one year are 

 not great, but they may become very thick in the course of centuries. 



