324 



cox. 



is wide. That the error is of the same order of magnitude regardless of the 

 method employed is shown in Table XV. 



Table XV.a — Effect of oven drying on Ihe mechanical composition of soils. — • 



Continued. 



[Numbers give percentages.] 



Soil. 



Air-dried.'' 



Dried at 110°. 



Num- 

 ber. 



Color. 



Depth. 



Sand. 



Very 

 fine 

 sand. 



Silt. 



Clay. 



Total. 



Sand. 



Very, 

 fine 

 sand. 



Silt. 



Clay. 



1 

 Total. 



10 

 11 



Dark broivn. 



Cm. 

 0-15 



15-30 



5.75 

 7.28 



4.82 

 6.43 



35.76 

 33.41 



54.27 

 .53.89 



100. 60 

 101. 01 



8.57 

 6.98 



12.86 

 19.45 



54.48 

 52.94 



24.71 

 21.45 



100.62 

 100.82 



^ Analyzed by A. S. Arguelles. All clay percentages were obtained by direct deter- 

 mination. 



>> Moisture separately determined and the results calculated to the dry basis. 

 " Subsoil ot No. 10. 



The above analyses were made simultaneously by the centrifugal method.-" 

 There was no attempt to have the size of the smaller grains in the different 

 fractions exactly conform to a given dimension, but rather to make them directly 

 comparable. It was thought that perhaps this could be more accurately done 

 if the samples were centrifugated each time for a definite number of minutes 

 rather than controlled by the use of a micrometer microscope, and this method 

 was used. The apparent clay in the two soils is reduced over 30 per cent by 

 drying at 110°. 



In all of the samples the analyses of which are recorded in Tables XIV and 

 XV there is a great tendency for the oven-dried soil to form hard aggregates 

 which do not disintegrate and in this way "to step up the percentages of the 

 coarser grains. This tendency is greatest for those soils containing the highest 

 percentage of clay. A. V. Bleininger -' found that clays dried to 150° were found 

 to show an extraordinary tendency to form hard aggregate particles and for the 

 clays to become coarser grained. "The difference between the normal and the 

 preheated clays, therefore, is due to coagulation; the material has become 'set', 

 to use the language of colloid investigators, and irreversible as far as practical 

 purposes are concerned." Bleininger- found, in 16 hours' soaking after heating, 

 that the general plasticity did not return and suggests heating as a practicable 

 method for correcting excessively colloidal clays. Therefore, it is evident that 

 mechanical analyses of oven-dried soils are not comparable since even duplicate 

 analyses of a clayey soil do not agree. The analysis must be made without 

 previous drying above ordinary temperatures. 



The degree of accuracy which may be obtained with the modified Sehone 

 apparatus on an air-dried soil disintegrated by shaking is sho^^^l not only by 

 the duplicate results of Tables XIII and XIV, but also by the following four 

 consecutive analyses of an air-dried soil made at widely separated times. 



'» Bull. U. 8. Dept. Agri., Bur. of Soils, loc. cit. 

 ^Tran. Am. Geram. Sac. (1909), 12, 504. 

 -Tran. Am. Geram. Soc. (1910), 11, 392. 



