THE PHYSICAL PEOPEETIES OF SOILS 55 



total surface of a mass of spheres is inversely proportional 

 to the diameter. 1 



If the particles were as small as those of sand say T Jo 

 of an inch the internal surface would be 3*1416 X 100 = 

 314*16 square inches in 1 cubic inch, equal to 418*88 square 

 yards in 1 cubic foot. 



' If the particles were as small as those of clay say ^"oo 

 of an inch the internal surface would be 3*1416 X 5,000 = 

 15,708 square inches in 1 cubic inch, equal to 20944 square 

 yards in 1 cubic foot. 



According to this calculation, the internal surface of a 

 cubic foot of clay is equal to a superficial area of over four 

 acres. In clay soils, as a rule, only some 30 or 40 per cent, 

 of the particles are as small as has been assumed above, 

 but, on the other hand, some of them are smaller. The 

 estimated amount of internal surface in a cubic foot of 

 different kinds of soil is approximately as follows : 



Square feet. 



Sandy soils 5,000 



Clay soils . 100,000 



Calcareous soils 200,000 



Humous soils 500,000 



Mass. The mass of a body means the quantity of matter 

 in it. It depends upon the density and volume. M=Vd. 

 Compared with water as unity, the density of most of the 

 rock-forming minerals lies between 2*5 and 3*5, but 

 magnetite, haematite and some others are heavier about 

 5*1. The density of the commoner rocks varies from about 2*5 

 to 3*0 ; that of clay and sand have already been given as 

 2*5 and 2*62 respectively, and that of peat as from 1*3 to 

 1*6. Humus, therefore, is the only constituent that affects 

 the density of soils to any considerable extent, and, with, 





