110 Pliysics of the Soil. 



determine the number in a -cubic inch. If a soil were made 

 up entirely of the largest size given in the last table, then 

 23 would build one edge of a cube an inch on a side and 

 the number in a cubic inch arranged in the manner repre- 

 sented in the upper part of Fig. 34. would be 



23 3 = 23 X 23 X 23 = 12, 167. 



On the other hand, if they were all the size of tEe smallest 

 grain in the table then the number would be 



25, OOO 3 _= 15, 625, 000, 000, 000, 



or enough to form three and a third continuous lines of 

 grains in contact from Boston to San Francisco. 



131. The Size of Soil Kernels. It must be kept in mind 

 that while it is true that the heavy clay soils are made up 

 largely of soil grains of the extremely small size considered 

 in (130) these minute grains are generally bound together 

 in groups or kernels of various sizes and it is only by long 

 boiling in water or thorough pestling that these can be 

 broken down. The writer has found that when air-dry 

 samples of the heaviest cla'y soils are thoroughly pestled in 

 the dry condition it is difficult to reduce their texture to a 

 finer degree than kernels averaging .01 to .005 m. m. in 

 diameter or such that from 2,500 to 5,000 are required to 

 span a linear inch; but even this degree of closeness of 

 texture is too fine to allow of proper drainage and soil ven- 

 tilation and to permit roots to make their way through tho 

 soil with the freedom required for good crops. 



132. Specific Gravity of Soil Grains. The specific gravity 

 of soil grains, or the number of times they are heavier than 

 an equal volume of water, varies somewhat, as does that of 

 the minerals which compose +hem. As there are not many 

 common minerals more than three times as heavy as 

 water and not many lighter than 2.5 times as heavy, the 

 specific gravity of soil grains will lie between these two 

 figures and it is usually found to be near 2.65. 



