172 The Pi. ant World. 



Besides the heat of wetting there have been proposed two 

 methods of measuring the internal surface of the soil. One of 

 these is the measurement of the amount of a given dye d- 

 sorbed from solution by a given quantity of soil. This has 

 proven of some utility in the classification of fine clays for 

 porcelain manufacture and similar uses. Otherwise it has 

 never been, nor is likely to be, of any importance. The third 

 and last internal surface method is the determination of the 

 hygroscopicity or the percentage of water absorbed by dry 

 soil from a saturated atmosphere. Essentially this is a measure- 

 ment of the vapor pressure of the water in the soil interspaces 

 and, as no experiments have ever been made specifically on this 

 point, no one knows whether this vapor pressure depends direct- 

 Iv upon the nature of the soil or not. Probably it does not 

 and probabh the determination of the hygroscopicity has 

 neither theoretical warrant nor practical utility. At any rate 

 the case for it is still unproven. 



llie IWiier Reicniivity Methods. Both the mechanical 

 analysis and the internal surface methods attempt to get at, 

 directly or indirectly, some fundamental, inherent property of 

 the soil, with which property its general physical nature is sup- 

 posed to be in close relation, This is perhaps not entirely neces 

 sary. If, for instance, there were some constant expressing the 

 physical relation of the soil to water and from which the water- 

 holding and water-moving properties of the soil could be deduced, 

 this would probably be all that is needed. Water relations are 

 so much more important than other physical properties that, 

 with thcni known, we could afford to forgo the rest. It is 

 probable, however, that any constant expressing satisfactorily 

 the water relations of a soil would express pretty nearly other 

 physical properties as well. All are probably dependant upon 

 the same, more fundamental, characters. 



Of the many suggestions towards some such "water con- 

 stant' ' the most elaborated is the method of Briggs and McLean 

 for the determination of the "water equivalent. "* This may be 

 defined as the percentage of water which will rema'n in a soil 

 when it is so whirled in a centifuge as to be subjected to a 

 force of definite and considerable intensity. Essentially this 



♦Bulletin 45 Bureau ot Soils, U. S. Department of Agriculture. 1907. 



