MINERAL CONSTITUENTS 



129 



standard conditions of temperature and pressure gives the data by 

 which the diameter is determined. The soil is placed in D, figure 

 71, a tube having a capacity of 100 or 200 c.c. with a wire gauze 

 bottom. This is connected by means of a tube to the aspirator A. 

 A cord with a weight attached exerts sufficient " pull " to draw the 

 air through the soil. The "effective diameter" is deduced by 

 means of a formula using the data determined. The flow of water 

 through the soil computed from the "effective diameter" obtained 

 corresponds very closely to that actually observed, as shown in the 

 table. 



Comparison Between Computed and Observed Flow of Water 



II. MINERAL SOIL CONSTITUENTS AND THEIR PROPERTIES 



1. Colloids. While the colloids of soils are usually classed 

 along with clay, their importance justifies separate treatment. Al- 

 though not as abundant in soils as many other constituents, yet 

 they possess such distinctive characteristics and impart these so 

 noticeably that they are of the greatest consequence not only from 

 a physical standpoint but from a chemical and biological as well. 

 Xon-colloids are called crystalloids. 



Colloids are substances composed of the very finest of particles 

 and when mixed with water appear to go into solution. When con- 

 taining a certain amount of water they appear jelly-like or gelati- 

 nous. Since the colloidal state is dependent upon the sixe of parti- 

 cles, it follows that many substances may exist in both colloidal and 

 crystalloidal forms. Up to the present time only about -100 sub- 

 stances have been found that exist in both. 



Examples of Colloids. The word colloid is derived from rolJa, 

 meaning glue. A glue or jelly-like consistency is one of the most 

 familiar characteristics of colloids. In the inorganic world almost 

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