152 



coalescing with, or being entangled -nith, the coarser particles of clay 

 and being carried doA^Ti by the latter. For this purpose, the clays 

 should not be too finelj- di\-ided, nor should they be highly jilastic, 

 as a somewhat coarser, porous material is more efficient and settles 

 more readily. It must, however, possess sufficient power of adsorption 

 to retain the finely suspended particles which cause the turbidity 

 in the fluid wliich it is desired to clarify. 



The use of clays as clarifj^ng agents is particularly successful m 

 the treatment of slightly oUy effiuents of spimiing works, wool- 

 scouring plants, distilleries, tanneries, dj-e-works, glue-factories, 

 breweries, and other industries producing an effluent containing 

 organic matter in a very ffiieh' diWded or coUoidal state. The best 

 results are obtained when the coUoidal matter in the effluent carries 

 a positive electric charge as it is dissipated bj- the added colloidal 

 matter bearing a negative charge. Domestic se^^age. however, is not 

 of this character, and, therefore, cannot be clarified in this manner. 

 This fact confirms the essentially coUoidal nature of the active 

 ingredient in clay used as a clarifjTng agent. 



The separation of water from clay in filter presses may, according 

 to F. Ulzer-^ be facilitated by inserting suitable electrodes in the 

 filter chambers, so as to coagulate the coUoidal material and yet 

 keep it from blocking the cloth. 



The extrusion of clay through dies is made more rapid by the 

 application of an electric charge, as described in '" British Association 

 Report on CoUoids, II.,"" 1918. 



The binding power of clays, Uke that of dextrin, gelatin, and other 

 weU-kno«ii coUoids is used for uniting other particles into a compact 

 mass as in the manufactiu'e of refractorj' materials from non-plastic 

 materials, '" lead " in lead pencUs, &c. Conversely, use is made of the 

 coUoidal property to reduce the excessive shrinkage of certam clays 

 by the addition of suitable non-plastic materials. 



In agriculture, the coUoidal jsroiJerties of clays play > .most important 

 part. It is now generaUy agreed that plants and .)ther organisms 

 growing in the soil, derive their sustenance chiefly from a film of 

 coUoidal matter surrounding the particles of inert material constituting 

 the bulk of the soil and occupying some of the interstices between 

 these particles. The soluble salts extracted from the mineral 

 material in the soil or subsoU, or added in the form of a fertUiser, 

 are adsorbed hy this fflm of coUoidal matter and transferred to the 

 plant-roots, &c. There are several different substances existing in 

 the coUoidal state in soUs, the more important being the coUoidal 

 " clay," siUca, alumina, ferric hj'droxide, and a mixture of partiaUj^ 

 decomposed organic substances coUectively kno^Ti as " humus." 

 The last-named acts as a j)rotective coUoid for other substances, and 

 absorbs manj^ soluble salts. In association ^^ith other coUoids, it 

 determines the amount of water retained by the clay. Humus is 

 negatively charged and is, therefore, coagulated into a gel by basic 

 substances, such as Ume. 



The colouring matter in swamps is generaUy positivelj' charged, 

 and is, therefore, precipitated by coUoidal clay. Alkaline humates 

 are not coUoids, but soluble substances. Being a negative coUoid, 



