56 



SOILS AND MANUEES 



the exception of vegetable soils, they do not differ much in 

 this respect. The true specific gravity is usually about 

 2*5 or 2*6. The mass of a given volume of soil, however, 

 cannot be determined from the density of the particles 

 because, in pulverent material, the space is not completely 

 occupied by them. 



The proportion of unoccupied space in soils varies con- 

 siderably, and the masses of equal volumes are therefore 

 often very different. The weight of a given volume of soil 

 divided by the weight of an equal volume of water is called 

 the apparent specific gravity. 



The volume weight and apparent specific gravity of some 

 soils and soil constituents are as follows : 



The difference between the apparent specific gravities of 

 sand and clay is greater than the difference between their 

 true specific gravities, because there is more unoccupied 

 space in the latter. In the case of the three samples of soil 

 the differences are due partly to the same cause and partly 

 to the larger proportions of humus which some of them 

 contain. The presence of stones increases the volume 

 weight of a soil because, being compact and impenetrable, 

 the apparent specific gravity of stones is the same as the 

 true specific gravity. The low r er layers of soil have, as a 

 rule, a higher apparent specific gravity than the upper 



