CHAPTER XXIV. 183 



mathematical demonstration of the reasons for this is Interesting, but 

 would scarcely be pertinent at the present time. 



Capacity, as previously stated, is the quantity of water which can 

 be introduced into a dry soil. It is usually expressed as a percentage 

 of the soil volume. The total quantity that a soil is capable of im- 

 bibing is termed its maximum capacity. This quantity is divisible 

 into two parts. The one removable by drainage, the other by evapo- 

 ration. The latter Is again sub-divisible into two parts. One part 

 is brought to the surface by capillary action and there evaporated, the 

 other is almost permanently retained within the soil, requiring for its 

 removal long continued application of heat. This is termed hygro- 

 scopic moisture. 



From German authorities we have the following maximum capaci- 

 ties of various soils, expressed in percentages of volume: 



Humus 70.3 per cent. 



Garden mold 69 per cent. 



Peat 63.7 per cent. 



Loam 60.1 per cent. 



Lime 54.9 per cent. 



Chalk 49.5 per cent. 



Quartz sand 46.4 per cent. 



For minimum water capacity, or water remaining after gravita- 

 tion, we have — varying according to depth — 



Humus, loose 43.6 to 49.1 per cent. 



Loam, fine, loose 38.4 to 48.3 per cent. 



Sand, loose 17.6 to 27 per cent 



The hygroscopic moisture was determined by Loughridge, of the 

 California experiment station, to be as follows: 



Clays from 2.6 to 14.5 per cent. 



Loams from 4.9 to 9.2 per cent. 



Sandy soils from .8 to 3.5 per cent. 



The wide range in these figures serves to illustrate the necessity 

 of experimenting directly with the soil itself, if exact data is re- 

 quired of any particular soil. 



The precise nature and mechanics of hygroscopic moisture are but 

 little understood. There is a good field here for independent inves- 

 tigation. 



Of the different capacities the hydraulic engineer is more par- 

 ticularly interested in that which relates to the quantity that may 

 be drained out; on the other hand, the aboriculturist is most inter- 



