224 PHYSIOGRAPHY 



in this respect, than the denser blanket of oxygen and nitrogen. 

 If it were thicker, it would be still warmer. If the amount of this 

 gas were doubled, it is thought that the temperature of high lati- 

 tudes would be notably increased, possibly enough to melt the ice 

 of Greenland. 



Water vapor. It has been noted that the water vapor in the 

 atmosphere is a variable quantity. It is all the time entering the 

 atmosphere as water vapor, and it is all the time being condensed 

 and precipitated from the atmosphere as rain, snow, etc., to be 

 again evaporated, condensed, and precipitated. Like much of 

 the C0 2 , it is making continuous rounds. The amount which the 

 atmosphere may contain at any time depends on temperature; 

 but other things, such as the available supply, help to determine the 

 amount which there is in the air in any one place. Like the car- 

 bon dioxide, the water vapor of the air helps to keep the earth warm. 



Dust. All the solid particles held in the air are dust. We do 

 not ordinarily see them except on dry, windy days, but dust from 

 the air is constantly settling everywhere, in doors and out, when- 

 ever the air is dry. Dust may be seen readily in indoor air if the 

 room is darkened and light allowed to enter through a narrow crack 

 or small hole. Even air which appears clear may in this way be 

 seen to contain countless particles of solid matter. The amount 

 of dust is sometimes very great, as over cities, and in dry and windy 

 regions. During the fogs of February, 1891, it was estimated that 

 the amount of dust deposited on roofs in and near London was six 

 tons per square mile. The variety of matter in the dust was great, 

 carbon (soot) being most abundant. 



Some years ago a method was devised for counting the dust 

 particles in a given volume of air. The result showed that in the 

 air of great cities there are hundreds of thousands of dust particles 

 in each cubic centimeter (a centimeter is less than four-tenths of 

 an inch) of air; and that even in the pure air of the country, far 

 from towns and factories, there are hundreds of motes per cubic 

 centimeter. It has also been estimated that "every puff of smoke 

 from a cigarette contains about 4,000 million separate granules 

 of dust." 1 The amount of dust in the air is greater over the 



'Mill, Realm of Nature. 



