III.] 



TRUE AND APPARENT DENSITY 



63 



giving the minimum pore space. If some small shot are 

 shaken into a graduated measure and the pore space 

 determined by pouring in a measured volume of water, 

 the indicated minimum will be found ; but if the experi- 

 ment be repeated with sand which has been sifted to get 

 approximately a uniform size, a higher figure will result. 

 In the one case the particles are too light to exert 

 much force towards the rearrangement of the mass ; in 

 the former case the heavy smooth shot slip straightway 

 into the most compact arrangement, because by it the 

 shot attain their lowest position. In consequence of the 

 pore space, the density of a soil in situ will differ very 

 much from that of the materials of which it is composed, 

 nor will all soils possess the same apparent density when 

 dry. Perhaps the best way of ascertaining the apparent 

 density of a soil or soil materials is to get a smooth 

 metal pint pot or like measure, fill it with the material 

 in question with gentle tapping, and then strike off the 

 upper surface smooth with a rule. The weight of the 

 contents divided by the volume gives the apparent 

 density, from which the true volume and the pore space 

 can be calculated, if the true density of the material be 

 known. The following table shows the true and ap- 

 parent density of the chief soil materials ; as a mean 

 figure for purposes of calculation, 2-65 can be taken as 

 the true density of ordinary soils : — 



The following table shows a few determinations 



