116 Physics of the Soil. 



the simplest method is to use a soil tube, represented in 

 Fig. 38, taking a number of cores of the desired depth, 



FIG. 38. Showing soil tube for taking samples of soil. 



drying them, and then compute the pore space with the 

 formula above. 



138. Largest Possible Pore Space. The largest possible 

 pore space in soils will be found in the cases where the com- 

 pound or kernel-structure is most marked. Referring 

 again to Fig. 34, imagine each sphere there represented 

 to be made up of other very much smaller spheres having 

 the same general arrangement. Were this the case it is 

 clear that in consequence of the compound spheres the soil 

 must have a pore space not less than 25.95 per cent, with 

 one arrangement and 47.64 per cent, with the other. But 

 in addition to this pore space there must be a like pore 

 space within each compound sphere so that in the first case 

 the total pore space would be 



25.95 + [25.95 per cent, of (100 - 25.95)] = 45.17 

 and in the second case 



47.64 + [47.64 per cent, of (100 - 47.64)] = 72 58 per cent. 



The first pore space, 45.17, it will be seen, lies close 

 to that possessed by the finer soils but the latter is larger 

 than anything ever found except it be in the loose mulches. 



The smallest pore spaces result when grains of different 

 sizes are so related that the small ones fall into the pores 

 formed by the large ones without at the same time crowd- 

 ing them farther apart. Referring again to Fig. 34, it 

 will be seen that if small spheres are packed into the pores 

 there shown, with the same arrangement that the large 

 ones have, the original 25.95 per cent, and 47.64 per cent 



