II.] THE NATURE OF CLAY 35 



of alumina which is formed by the weathering of fel- 

 spar ; the other materials present consist of extremely 

 fine grains of quartz and other weathered minerals, 

 together with more or less oxide of iron. "China 

 clay" and the best "pipe clays" contain little or no 

 iron ; the deep-seated clay formations are generally 

 coloured dark green or blue or black by the presence ' 

 of ferrous silicates like glauconite ; on weathering and 

 exposure at the surface the clays become yellow or 

 brown, owing to the oxidation of these ferrous to ferric 

 salts. 



Water in which a little clay has been rubbed up 

 remains turbid for a very long time ; days and even 

 weeks elapse before the particles settle down to the 

 bottom — indeed, however long the liquid may be at 

 rest, a slight haze or cloudiness may be observed 

 within it Schloesing has drawn a distinction between 

 the part of the clay, amounting to 1 or 2 per cent 

 only of the whole, which persists in remaining sus- 

 pended and the portion which settles down ; he has 

 called it "colloid clay," and attributes many of the 

 typical clay properties to the jelly-like medium of 

 colloidal matter by which the other defined particles 

 of the clay are surrounded. Schloesing associates this 

 colloid clay with such typical colloids as the highly 

 hydrated forms of silica and organic bodies like starch 

 and gum which, though they appear to be truly dissolved, 

 yet cannot diffuse through a membrane, and form, on 

 drying, hard non-crystalline masses, with much shrinkage 

 and a characteristic fracture. But later researches on 

 colloids show that they are not essentially different 

 from suspended matter; they consist of particles too , 

 fine to settle down in water, or to be arrested by a 

 filter even of porous porcelain, but which are still 

 sufficiently coarse to show their presence when a strong 



