PERCOLATION 89 



(Sachsse and Becker's experiment, p. 33), and by the forma- 

 tion of compound particles. The resistance of puddled clay 

 to the passage of water is due in great part to the extreme 

 fineness of the particles, and to the great resistance which 

 has to be overcome in passing between them ; but there can 

 be little doubt that a considerable part of the difficulty is 

 due to the colloid constituent of the clay, which occupies 

 the interspaces with a jelly-like substance, and thus immensely 

 increases the resistance offered to the passage of water. The 

 coagulation of this colloid profoundly alters the character 

 of the clay. 



In the case of a heavy loam or clay soil, under natural 

 conditions, percolation is much facilitated by the presence of 

 channels formed by worms, or by the roots of plants ; and 

 by the occurrence of fissures, either originating in times of 

 drought or natural joints in the formation. Bain may pass 

 down these passages before the soil is saturated ; summer 

 drainage on such soils is often chiefly of this character. 



The amount of water passing through a soil is measured 

 by means of drain-gauges or lysimeters ; an instrument of 

 this kind was constructed by Dalton in 1796, and the same 

 method of investigation has since been employed by many 

 observers. Most drain-gauges consist of cylinders or square 

 frames artificially filled with soil, and usually 3 ft. deep, with 

 an arrangement below for collecting and measuring the water 

 which passes through. The three drain-gauges at Rothamsted, 

 constructed in 1870 (Jour. Roy. Agri. Soc. 1881, 269), con- 

 sist of rectangular blocks of undisturbed soil, isolated by walls 

 of brick set in cement, and supported below on perforated 

 iron plates, below which is placed a metal funnel, so that 

 all drainage water can be collected and measured. The three 



