154 SOILS: PROPERTIES AND MANAGEMENT 



colloidal material does not differ from crystalloidal 

 in chemical composition, but the distinction is merely 

 one of size of par ticles. For example, if large particles 

 are suspended in water, they will immediately sink, 

 since their weight is so much greater than the sur- 

 face that is exposed for buoyance. When these particles 

 are decreased in size, their weight decreases much faster 

 than the surface exposed. It is therefore evident that 

 a point will at last be reached at which the particles, 

 because of their minute size, will form a homogeneous 

 solution The upper limit of the colloidal state has then 

 been entered. 



103. The colloidal state. — The colloidal state in which 

 these particles are now found is a peculiar one, and ex- 

 hibits much diversity, not only in properties, but also 

 in the size of particle in which the material exists. The 

 upper limit of the clay group as designated by the classi- 

 fication of the United States Bureau of Soils is .005 milli- 

 meter, while the upper limit of the particle existing in a 

 colloidal state is estimated to be below .005 of a micron, 

 or .000005 millimeter. Indeed, so small are the colloidal 

 particles that they become molecular complexes, that is, 

 a few molecules may go to make up a particle. 

 The various colloids, or the same colloid under different 

 conditions, may exhibit greatly differing sizes of particles. 

 Some colloidal particles are very large, approaching the 

 upper limit already set for material in such a state. Other 

 particles are finer. It is evident that a gradation must 

 exist until a particle is reached which consists of only one 

 molecule. The solution then ceases to be a molecular 

 complex and becomes a true solution. The colloidal 

 state thus grades into the true solution, just as an 

 ordinary suspension grades into a true, or colloidal, 



