ii.] THE NATURE OF CLAY 39 



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 was the first to draw a distinction 

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

 cent, only of the whole, which persists in remaining 

 suspended and the portion which settles down ; he has 

 called it " colloid clay," and attributed the typical clay 

 properties to the jelly-like medium of colloidal matter 

 by which the other defined particles of the clay are 

 surrounded. These colloids of clay may be associated 

 with such typical colloids as the hydrates of silica and 

 organic bodies like starch, gelatin and gum, which will 

 pass into a kind of solution the " sol " condition when, 

 however, they cannot diffuse through a membrane. 

 Under certain conditions of temperature, or in the 

 presence of electrolytes, their relation to the water by 

 which they are surrounded is changed, and they set 

 to a more or less firm jelly, the " gel " phase, and when 

 they are then dried they form a hard non-crystalline 

 mass with much shrinkage and a characteristic fracture. 

 In their "gel " condition they will imbibe considerable 

 quantities of water and swell. The colloids in their 

 " sol " condition differ only from suspended matter in their 

 fineness of grain and the intimacy of their contact with 



