I] NATURAL HISTORY OF THE SOIL 5 



drying and the hard clots that result, its marked 

 alteration after treatment with lime, are all mani- 

 festations of the special properties of the fine material. 

 These properties are characteristic of the jelly-like 

 condition, technically known as the colloidal state, 

 into which many substances can be brought. 



These properties are not shown by the coarse 

 material ; a sandy soil (in which the coarse particles 

 predominate and the fine particles do not form more 

 than 5 10% of the whole) has no great power of 

 absorbing water and therefore readily dries, it is not 

 sticky, does not shrink on drying or form hard clods, 

 and undergoes no obvious physical change after 

 treatment with lime. 



It must not be supposed that any hard and fast 

 line can be drawn between the coarse sand material 

 and the fine clay. One shades off imperceptibly into 

 the other, and so gradual is the transition that the 

 special name silt is used to designate the intervening 

 material. The lines separating sand from silt on the 

 one hand and silt from clay on the other are purely 

 conventional and are agreed ii]K>n by soil chemists 

 in each country, but unfortunately no two countries 

 accept quite the same definitions. In Great Britain 

 clay is defined as any material the particles of which 

 are less than '002 mm. in diameter (i 3^000 m -) an( l 

 silt as any material the Articles of which are above 

 this but below '04 mm. in diameter (^ 7 in.): in the 



