The Soil 



due to this. Even in the temperate regions, when the 

 nights grow much longer than the days, we find that we 

 approach the arctic conditions; and when the days grow 

 longer and the nights are shorter, we approach the con- 

 ditions of the tropics, where days and nights are even 

 throughout the year. 



It is thus clear that the heat produced by 

 of Sunshine ^^^ action of the sunshine on land and water 

 becomes the great agency in producing plant 

 life on the earth. We must realize that the rays of the sun 

 move through a vast space of intense cold, and that heat 

 is a transformation of the motion made here on earth, 

 and its intensity depends largely on the conformation of 

 the earth itself. Down about the sea level, where the pres- 

 sure, and consequently the density, of the atmosphere is 

 greatest, the radiation is slower and heat is retained longer, 

 while on the mountain top the lesser atmospheric density 

 permits of a more rapid radiation; so that on the more 

 lofty mountains we have arctic conditions of eternal snow 

 and ice. On the surface of the sea the rays of the sun set 

 up a rapid motion in the molecules of the water, making 

 them warmer until they are changed into a condition of 

 vapor and fly off to form clouds, which distribute the rain 

 on the earth. 



We can now see how the disintegrating effect of the heat 

 produced through motion from the sun affected the rocky 

 surface of the earth, and the coming of the clouds from 

 the ocean washed to the lower levels the loosened particles 

 and started the first formation of soil. 



Then came the work of the atmosphere itself. At 

 the sea surface the pressure is about fifteen pounds to 



