APPENDIX 591 



apart are not sufficient. Every colloid is a heterogeneous system 

 ill which there is a continual offort loward establishing equilibrium. 



Fundamental Problem of Colloid Chemistry. — Kruyt says that 

 llie fujulniuciital ]U'o])lein of colloid chemistry is to ascertain the 

 reason for the inability of the surface tension to unite the particles. 

 In the case of suspensoids it seems that the surface charge and dif- 

 ference in potential are fundamental factors in keeping the ]iar- 

 ticles apart, while the orientation of surface molecules is funda- 

 mental in determining the cohesive or adhesive forces. 



In emulsoids, conditions are a little more complex and a third 

 factor perhaps ranfe first in importance. Attention has previously 

 been called to the difference between emulsoids and suspensoids. 

 In emulsoids, the dispersion medium forms an intimate relationship 

 with the dispersed material. Since the emulsoids encountered in 

 immunity are made up of water as the dispersion medium and 

 protein the dispersed substance or phase, the water is in intimate 

 relationship with the dispersed particles of protein. It is con- 

 sidered that each particle has an intimate w^ater envelope around it. 



Factors in Emulsoid Stability. — The factors involved are the 

 particle and its surface, the water layer and the electrical double 

 layer. Anything that reduces this water layer to a certain 

 critical amount increases the cohesive properties. The lowering 

 of the potential to the critical point favors precipitation and also 

 cohesion, and as mentioned above the nature of the material of 

 the surfaces is an additional factor. The water envelope is of 

 sufficient importance to prevent precipitation even when the 

 critical potential is reached. This is illustrated by Kruyt (Fig. 

 27). 



Precipitation by Electrolytes. — It will be seen from Fig. 27 that 

 if the charged emulsoid is dehydrated, it becomes a charged sus- 

 pensoid that can be precipitated by electrolytes neutralizing the 

 charges. If the charge on the emulsoid is removed by elec- 

 trolytes, it remains an emulsoid. but without a charge, and can 

 be precipitated by dehydrating it with alcohol. Thus it will be 

 seen that the neutralization of charge causes ])recipitation or 

 coagulation of suspensoids but not of emulsoids. 



Macleod says that ''a (juantity of electrolyte which is capable of 

 producing complete precipitation when added all at once to sus- 

 pensoids will be ineffective when added in small quantities. This 



