HYGROSCOPIC WATER 61 



the coarsest sand was the one absorbing only 0-79 per cent, of 

 water. As the proportion of clay increases the absorptive 

 power for water- vapour rises. In one case the whole soil, and 

 some of the separated portions from its mechanical analysis, 

 were individually tested as to their power of absorbing 

 wa ter- vapour ; the percentages of water expelled at 200 C. 

 were as follows. 



Whole Soil. Separated Portions. 



Clay. Hydraulic Value in millimetres per second. 



< 0-25 ... 0-25 ... 0-50 

 5-34 ... 17-60 ... 7-96 ... 2-91 ... L73 



The various fine constituents and colloids classed as clay 

 had thus by far the greatest power of absorbing water, and 

 the coarsest constituents the least. 



The connexion between the percentage of true clay in a 

 soil and its absorptive power is not a very close one ; clay, 

 in fact, is not the constituent of soil possessing the highest 

 absorptive power ; the purest clay experimented with (the 

 pipe-clay of Table I) absorbed in fact only 9-09 per cent, of 

 water. The constituents of soil having the highest absorptive 

 power are humus, ferric oxide, and the hydrated silicates 

 decomposable by acids ; if either of these is present in con- 

 siderable" quantity, the soil will have a high absorptive power 

 irrespective of the proportion of clay it may contain. In 

 cultivated soils the hygroscopic power is determined most 

 frequently by the proportion of humus present. 



The soils examined by Loughridge having the highest 

 absorptive power for water- vapour were some from the Sand- 

 wich Islands; these soils were derived from the decomposition 

 of lava, and were extremely rich in hydrated silicates, in 

 hydrated alumina and ferric oxide, and in some cases humus. 



