CLARIFICATION AND DECOLORIZATION 235 



in size they eventually merge into colloidal sols which are 

 sometimes analogously named " suspensoids " and " emul- 

 soids," if further reduced in size into " suspensides " and 

 " emulsides," and with further reduction into true solutions. 

 On this view not only suspensions and emulsions, bat also 

 sols, solutides and solutions are all heterogeneous. Now in 

 practice the clarifying of a gelatine sol involves only the 

 removal of the particles which are evident to sight. What 

 is needed is that the product should make a sol or gel which 

 to the naked eye appears to be optically clear both to 

 reflected and to transmitted light. If desired, the limit 

 could be expressed in terms of dispersity or specific surface. 

 Now it is a comparatively easy matter to remove the coarser 

 substances which often pass into the sol, e.g. undissolved 

 portions of raw material or the insoluble portions, such as 

 the hair, the grain (hyaline layer), and the elastic fibres of 

 skin gelatine material, and the fibres which even remain in 

 extracting acidulated bones. A more difficult proposition 

 is the removal of still finer particles which may be almost 

 said to be in colloidal solution, but which at any rate are so 

 large that they cause a visible opalescence or even a turbidity 

 of the gelatine sol. A more difficult task also is the removal 

 of minute particles of grease, which are an exceedingly 

 common cause of turbidity and which are often very 

 effectively emulsified in the sol. 



Now at this stage it is necessary to point out that besides 

 the difference in the size of the particles of the disperse 

 phase, there is another important difference involved, viz. 

 that the particles of a colloid sol carry an electric charge 

 owing to the adsorption of electrically charged ions of the 

 electrolytes (salts, acids or alkalies) present. If this charge 

 be removed the colloid is precipitated (coagulated, floccu- 

 lated) and is then filtered off with comparative ease. This 

 precipitation can be brought about by a reduction or 

 elimination of the potential difference between the disperse 

 phase and the continuous phase. The electric charge given 

 by the adsorbed ions may be reduced by dilution, for dilution 

 causes a lessened adsorption of the charging ions. Hence 



