POKOSITY OF SOILS. 25 



greater will be the amount of water absorbed, because 

 the total surface of the particles is greater, and the longer 

 will it be retained. Thus a soil consisting of coarse 

 gravel will not retain water. A soil of pure quartz sand 

 will absorb but a small quantity, and will soon part with 

 it, while a fine alluvial soil will absorb a large amount, 

 and retain it a long time. The following table gives the 

 results of experiments made by Schtibler, to determine 

 the capacities of different soils for water and their com- 

 parative power of retaining it. In these experiments the 

 different soils were thoroughly wetted with water up to 

 the point of saturation, and the increase of weight noted; 

 this is shown in the first column. In the second column 

 are given the quantities of water which evaporated in 

 four hours, the samples of soil being spread over equal 

 surfaces. 



Per cent of water Per cent of water 

 absorbed. evaporated in 4 hours. 



Quartz sand 25 88.4 



Limestone sand 29 75.9 



Clay soil (40 per cent sand) 40 52.0 



Loam 51 45.7 



Common arable land 53 32.0 



Heavy clay (20 per cent sand) 61 34.6 



Fine Carbonate of lime 85 28.0 



Gardensoil 89 24.3 



Humus (peat or decayed vegetable matter)181 25 .5 



Thus the greater capacity a soil possesses for the ab- 

 sorption of water, the longer it retains it. It is obvious 

 that upon this depends to a very great extent the quanti- 

 ty of water that will be needed for the irrigation of any 

 particular soil. Before any calculation, as to the needed 

 supply, can be made, this point will have to be duly con- 

 sidered and determined by the irrigator or hydraulic en- 

 gineer. The difference arising from the variations in the 

 texture and composition of soils has been closely studied 

 by the French irrigator s and engineers. M. Gasparin, 

 who stands at the head of the numerous writers upon 

 this subject in that country, states that a soil which con- 



