32 THE AIR. 



dous falls of rain in consequence (fig. 2). When the clouds 

 are overcharged with moisture in cold climates or in cold 

 weather, the vapour freezes as it condenses and forms snow, 

 which under the microscope presents a series of the most 

 beautiful star-like cystals (fig. 3) ; the same result is 



FIG. 3. 



produced by the freezing of the vapour or fog near to the 

 earth's surface, this is called "hoar-frost," which is (like 

 dew) deposited on those parts of the surface most cooled 

 by radiation. Hail is caused in all probability by the drops 

 of rain passing through a cold stratum of air and becoming 

 frozen as they fall. There are many wonderful accounts on 

 record of great masses of ice falling from the clouds, some 

 of them several feet thick ; the cause of these phenomena 

 (if they ever did occur) is not known. The curious occurrence 

 of red or green snow, which has sometimes been known to 

 fall, is owing to the snow being mixed with myriads of 

 minute cellular plants called the Protococcus pluvialis, which 

 in one state of its existence is green and in another red, and 

 they colour the snow accordingly. The cause of their 

 sudden accumulation in such vast quantities is not known, 

 but may be sought for in some peculiar state of the atmos- 

 phere favourable to their growth, which in many places is 

 prodigiously rapid. 



The temperature of the air differs very greatly in different 

 situations at the surface of the earth, the extreme difference 

 being about 180 degrees ; thus in winter, at the poles, the 

 thermometer frequently stands at 60 degrees below zero, 

 and at the equator, in summer, 120 degrees above, but, in 

 the higher regions of the air, it is even colder than at the 

 poles, for the sun shining on the earth heats it, and the air 

 is heated by contact with it ; this is its only source of heat, 



