Studies i.\ Soil Physics, IV. 175 



researches where labor and expense are minor considerations. 

 Whether this condition is hopeless, we shall see jiresently. 



Co)tc/usioiis. In the preceding j^ages there have been dis- 

 cussed four '"constants'' whicli have been used or suggested: 

 (I) the mechanical analysis; (.'.) the heat of wetting; (3) the 

 moisture equivalent; and [4) the critical moisture content. Of 

 these the water equivalent is purely and entirely empirical 

 both in method of determination and in its correlations with 

 other properties. It does not attempt to measure any constant 

 propertv of the soil, but onlv the reaction of the soil to a set of 

 arbitrarily chosen conditions. The mechanical analysis is 

 similarlv empirical in its correlations with other properties, 

 but not so much so in its methods. The mechanical compo- 

 sition is a definite and constant property of the soil and is so 

 far as tlie mechanical analysis actuallv measure; this compo- 

 sition, in so far is it a soil constant. But this is not very far. 

 E^ecause of the group error above discussed the mechanical analy- 

 sis is never more than a moie or less accurate approximation 

 to the real mechanical composition, and it is usually less accurate 

 rather than more. I'urthermore the mechanical analysis is 

 expressed in complex and distinctly unserviceable numbers. 

 The heat of wetting is a little better oiT. It is similarly empir- 

 ical in correlation and similarly rational in determination (since 

 it tries to measure a real soil property — the internal surface) 

 It surpasses the mechanical analysis however, in that it actually 

 measures the things which it aims to measure and does it pretty 

 accuratelv, even if not directly nor in absolute figures. But 

 from the logical viewpoint the critical moisture content is the 

 best of all, since it is rational not only in method of determina- 

 tion but also in some of its correlations with other properties. 

 In relation to phvsical condition the value of the critical mois- 

 ture content means something immediately, directly, and in 

 itself regardless of what further correlations may be estab- 

 lished empirically. 



But logical correctness is not the whole of the matter. Prac- 

 tical utilitv is the essential criterion and here the critical 

 moisture content (because of the difiiculty of its determina- 

 tion) is woefully lacking. Taking all matters into account 

 one must conclude that the heat of wetting is the best of the 



