122 



sulphate, ammonium urate, aniline, ethylamine, methylamine. 

 Here, again, some of these substances may have to be transferred 

 to the second (or first) group if greater (or less) amounts than 

 those mentioned are used. 



(5) Substances which have no appreciable effect on the slip : 

 e.g., alcohol. 



Rieke has stated that the most soluble substances increase the 

 viscosity of the slip, but their effect may be neutralised by the 

 addition of a solution of barium hydroxide. The most harmful 

 sulphates according to the same investigator are those of calcium, 

 aluminium, and the heavy metals. Alkali sulphates stiffen the slip 

 when only • 1 per cent, is present ; larger proportions render it thinner 

 until 1 per cent, is reached, after which they stiffen it again. Zinc 

 and coj)per sulphates exhibit this phenomenon of variableness to a 

 marked degree. 



Bleininger found that the first addition of clay (up to 3 per cent.) 

 decreased the viscosity of water on account of the deflocculation 

 of the clay by dilution and the solution of the contained electrolytes. 

 When, however, the addition of clay became so great that no further 

 matter went into solution and the effect of the gel showed itself, the 

 viscosity increased with each addition of clay. This negative viscosity 

 is peculiarly characteristic of some clays. 



The size of the particles of the purer clays is comparable with that 

 of coUoidal particles, but most clays contain so large a proportion 

 of larger particles that it is almost impoiisible to isolate those which 

 are colloidal, in an entirely satisfactory manner. 



The adsorptive power of clays bears a strildng similarity to that 

 of colloids, or rather to that of a mass of inert material, the particles 

 of which are covered with a film of colloidal matter which also fills 

 some of the interstices. Thus, clays adsorb soluble dyestuffs, tannin, 

 humus, oil, grease, salts,* &c. ; and Hirsch and others have found 

 that barium, lead, and aluminium salts are adsorbed more readily 

 than those of Ume and magnesia. Chlorides and nitrates are adsorbed 

 more than sulphates, but alliali salts with the exception of the alkaline 

 carbonates are not adsorbed. The behaviour of the alkaline carbonates 

 may be explained by the almost invariable presence of calcium ions 

 in clays, which react with the carbonate forming a precipitate of 

 calcium carbonate, and so removing the carbonate ion from solution. 

 Rohland^ states that some clays wliich are only moderately plastic 

 may, on the addition of alkali and certain salts, or through some 

 chemical change, be made more adsorptive. The adsorptive power 

 of clay is valuable in some industries, and it is on account of this 

 power that if clay is mixed with neutral or slightly acid muddy 

 solutions or emulsions, when the clay settles it will be found to leave 

 a clear Hquid. The adsorption of a clay is usually determined by 

 noting the loss of colour of a dye solution such as malachite green, 

 and comparing it with another similar solution to which a standard 

 clay has been added. 



* Many clays retain salts so tenaciously that it is impossible to wash them 

 clean with plain water, but they can be removed by washing with a solution of 

 a salt which is more readily absorbed by the clay. 



