153 



humus is precipitated by cations such as Ca, Fe, or Al which alter its 

 permeability, absorption, and the way in which a soil " works." 

 Fertilisers convert the silica gels into sols which rise by capillary 

 action, and are again gelated nearer the surface of the ground or 

 in the cells of the plants which absorb them. For further details, see 

 "British Association Report on Colloids, II.," 1918, pp. 70-81. 



In the manufacture of detergents, the use of clay has been 

 developed to some extent on account of its colloidal properties, apart 

 from its use as a filler. 



It is well known that the amount of active colloidal material in 

 ela\'s can be increased by the addition of a certain proportion of 

 alkali, which disperses the particles of clay, as shown on p. 119. 

 F. E. Weston^* made use of this fact in order to bring china clay 

 into a highly colloidal state, and claimed that the colloidal clay thus 

 formed could be used as a substitute for soap. His results are not, 

 however, conclusive, and the clay-soap can only be used in special 

 cases. It is of little value in toilet preparations, but as a substitute 

 for the coarse soaps used in wool scouring it has been used with 

 success, and several preparations are now on the market. 



The chief advantages claimed for colloidal clay soap are that it 

 absorbs dirt and grease, and removes them without any deleterious 

 eflfect on the material, there being no injurious chemicals in the 

 preparation, and that it is capable of removing unsaponifiable oily 

 substances — a result which is not possible with the ordinary scouring 

 media. It is stated to have a greater detergent power than ordinary 

 soap and its antiseptic properties may be of considerable value. 



It has been shown by Bancroft, the organic soaps depend partly 

 on their solubility and decomposition (hydrolysis) for their detergent 

 projierties, and as siich properties are not possessed by clay soap, 

 they have a disadvantage in this respect. 



As yet, the colloidal properties of clay have not been widely 

 investigated ir this direction, and a very large field may be opened 

 out for the us of pure clays in soap manufacture, though the cost 

 of purifying brick, tile and other clays — even though they are more 

 plastic and contain more colloidal material — would, in most cases, be 

 prohibitive. 



References. 



^ Seger, Gesamin., Schriften., ed. Dr. H. Hecht and E. Cramer (Tonindustrie 

 Zeitung, Berlin, 1908). 



* Asch, W. & D., The Silicates in Chemistry and Commerce (Constable & Co., 



London). 

 ^ Martm, G., Chemical News. 



« Schloesing, Compt. Rend., Ixxix., 376-380, 473-477 (1874). 

 5 Rohland, P., Die Tone. 



* Searle, A. B., An Introduction to British Clays (Griffin, London, 1911). 

 ' Ashley. Trans. Amer. Cer, Soc, xii., 768 (1910). 



* Schurecht, Journ. Amer. Cer. Soc, 443-4.50 (1919). 

 ' Kosmann, Tonindustrie Zeitung, 352 (1895). 



i» Zebiseh, Sprechsaal, 1028 (1894). 



" Mellor, Green & Baiigh, Trans. Eng. Cer. Soc, vi., 161-170, 1906. 



1^ Ostwald, W., Colloid Chemistry. 



