168 The Plant World. 



thing to think with. The mind must not carry one quantity 

 but several and compare them jointly and severally (but all at 

 the same time)with that external property whose correlation 

 with mechanical composition is being sought. The requisite 

 degree of mental dexterity is not common. Furthermore the 

 lack of unity in the expression of mechanical analysis precludes 

 the application to it of curves and of graphic methods generally — 

 a loss, the great proportions of which can be partially appreciated 

 by recalling the immense service which these methods have 

 rendered in other branches of physical science. 



Closely related to this fault of disunity in expression is 

 the error introduced by the fact that the particles within the 

 limits of any one of the size groups are necessarily lumped 

 together in the result. From the mechanical analysis we learn 

 merely that so much material is within the size limits of a given 

 group. We do not know whether the particles comprising it 

 are of sizes mainly near the upper limit or are mainly near the 

 lower, or whether they are more or less uniformly distributed 

 among the intermediate sizes. W^ith finer soils this error is of 

 great practical importance. For instance, according to the 

 usual American system, all particle 3 below .005mm. in diameter 

 are classed as ' ' clay ' ' though in one soil this ' ' clay ' ' may consist 

 mainly of particles just inside the limit, (say .002 mm. to .004 

 mm. in diameter) while in another it is composed largely of 

 particles so small as to approach the colloidal state. The 

 physical properties of two such soils are radically different but 

 their mechanical analyses are exactly the same so long as the 

 same quantity of material lies below the .005 mm. limit. 

 Exactly this case is of such frequent occurrence in pracf.ce that 

 various attempts have been made to devise methods for the 

 determination of the ' 'colloidal clay" as an adjunct to the ordi- 

 nary mechanical analysis. These methods have not been 

 particularly successful, but even if they were the solution 

 of the difficulty would be but partial. It would amount to no 

 more than the addition to our present system of size limits of 

 another one below our present lowest. Within the new groups 

 thus established the difficulty would be nearly as great as be- 

 fore, and the slight improvement in accuracy which might be 



