G. D. Hubbard— Colloids in Geologic Problems. 103 



movements of the waters help to keep the particles up, 

 but in spite of the movements, electrolytes will bring them 

 down, so it seems probable that the electric conditions are 

 as significant in suspensions as is the agitation of the 

 water. If the particles of the colloidal material, and 

 other substances in the solution are similarly charged the 

 tiny pieces of the colloid do not fall. 



Even when waters are allowed to stand as still as they 

 can in a laboratory, some of these colloidal suspensions 

 do not clear up in months. It is believed that the parti- 

 cles are so small that their own Brownian motion pre- 

 vents them from clinging together into flocculent masses 

 large enough to fall through the water. Stokes' formula 

 gives the following relation: — 



V = 2r 2 g/9k.(d — d') 



V is the velocity of fall of spherical particles of radius r ; 

 k is the viscosity of the fluid ; d' and d are the densities 

 of the fluid and the particle respectively, and g the accel- 

 eration due to gravity. If water and the same kind of 

 particles are used on the same table for experiment, the 

 variables all drop out but r. 



Theoretically, then, the rate of fall can not quite be 

 zero, and permanent suspension becomes impossible if 

 the particle is denser than the medium, but practically, 

 since V varies with the square of the radius, we can find 

 particles so small that their fall would be so slow that 

 convection currents and Brownian movement would be 

 ample to keep them up. When two or more coagulate 

 together, their Brownian kicks are proportionately much 

 smaller, and the particle begins to" descend. Protecting 

 films of gelatine or other substance which does not mix 

 readily with the medium may surround each suspended 

 particle and prevent its cohering or coalescing with others 

 into masses large enough to fall. Any influence then, 

 Brownian motion, similar electric charge, or protecting 

 films, may help keep the particles small and aid them in 

 staying in suspension. 



Colloids as Cements. — It has long been known that 

 nothing soluble occurs in the shales to hold them together, 

 and yet they are extremely resistant to pulling strains. 

 We say their tensile strength is high. The ultra clay in 

 shales and clavs has been shown bv ceramists to consist 



