54 SOILS AND MANURES 



amount of interspace, but that which affects it most is the 

 arrangement of the particles. 



The interstitial space in dry soil is filled with air, but the 

 air may be wholly or partially displaced by water. The 

 magnitude of the individual spaces may be judged from 

 the size of the particles. 



The total amount of unoccupied space can be calcu- 

 lated from the formula 



*= 100 -(!>*). 



S is the percentage of interstitial space, M the mass of 

 soil, V the weight of an equal volume of water, and d the 

 true density of the soil. 



Calculating from these data the following results were 

 obtained : 



UNOCCUPIED SPACE IN DRY MATERIAL. 



Per cent. 



Quartz sand . 44*7 



Clay ' . 59-5 



Humus 75-0 



Arable soil ......... 53'4 



Old pasture , 64-1 



Internal Surface. The internal surface of a soil is 

 simply the sum of .the surfaces of all the particles. It 

 depends upon the shape and size of the particles. 



The surface l of a sphere of unit diameter is 31416 

 square units. With spheres of smaller diameter there 

 would be a greater number in a given volume, and the 

 total surface would be increased. Thus, if the diameter 

 were divided by a there would be a 3 times the number and 

 the total surface would be a times 3'1416. In short, the 



1 The surface of a sphere is found by the formula s = 4 TT r 2 . 



