CLARIFICATION AND DECOLORIZATION 237 



aim at filtering a gelatine sol when it is near the iso-electric 

 point, which is stable enough for gelatine itself, but a point 

 of instability for many undesired impurities. Yet another 

 phenomenon of colloid chemistry is concerned, viz. " pro- 

 tection." The particles it is desired to precipitate not only 

 adsorb ions of electrolytes, but also the gelatine sol itself, 

 and the particles, thus covered by a layer of a stable emulsoid 

 sol, attain much of the stability of this gelatine sol. Un- 

 fortunately for gelatine manufacturers, gelatine possesses 

 very great powers as " protective colloid," and this no 

 doubt greatly enhances the practical difficulty of obtaining 

 a clear and bright sol or gel. Here again dilution of the sol 

 reduces the adsorption and correspondingly reduces, to some 

 extent, the difficulty. 



With regard to the turbidity or opalescence in a gelatine 

 sol due to minute globules of grease, the case presents some 

 analogy to the coarser colloid solutions, but the analogy has 

 its limits, for an emulsion of grease is not an emulsoid sol. 

 Doubtless the grease globules exhibit adsorptive phenomena, 

 in which case the valency rule comes into force ; the gelatine, 

 also, by lowering interfacial tension, assists in protecting the 

 emulsion ; but grease emulsions are certainly stabilized in 

 alkaline media (hence the detergent effect of soap, soda, 

 borax, etc.), and it is undoubtedly easier to separate the 

 emulsion by making the medium acid. Hence the practical 

 fact that an acid sol is more easily clarified from grease than 

 an alkaline or even than a neutral one. 



The next stage in clarification is the separation of pre- 

 cipitated matters and of the coalesced particles of grease. 

 This may be attained by the two processes usual. in such a 

 problem of chemical engineering, viz. sedimentation and 

 filtration. After precipitation, therefore, the sol should be 

 allowed to stand for some hours, during which time the 

 precipitate not only flocculates but also settles to the bottom, 

 and the globules of grease coalesce further and rise to the 

 top, from which they may be skimmed off. Sedimentation 

 alone is both too slow and too incomplete to be sufficient 

 for proper clarification, and in these days it is always 



