COLLOIDS 501 



tion or sol. 3 Albumin, gelatin, and agar may be obtained crystalline 

 by proper regulation of temperature and the use of proper solvents, 

 as solutions of ammonium sulphate for albumin, and alcohol-water 

 mixtures of varying strengths for the two latter substances. Sodium 

 chlorid has been obtained in the colloidal condition by precipitating 

 it in a solution of sodium sulphocyanate by hydrochloric acid, each 

 of the reacting substances being dissolved in a mixture of amyl alco- 

 hol and ethyl acetate. 



There is much evidence that leads to the belief that all colloid 

 systems are unstable. Van Bemmelen characterized them as systems 

 which never reached a state of rest, that is, were never in equili- 

 brium. The conditions which determine the appearance of a body 

 in the colloid or crystalline form lead to the suspicion that bodies 

 always separate from solution in the amorphous or colloidal condi- 

 tion and that all crystallization is a secondary phenomenon. The 

 conditions that are favorable for the transformation of a colloid into 

 a crystalline form are a considerable solubility and a considerable 

 rate of crystallization. Where either or both of these is at 

 a minimum the conditions are favorable for relative permanence 

 in the colloid condition. It is upon the basis of this prin- 

 ciple that von Weimarn succeeded in obtaining relatively stable 

 colloidal solutions of common salt and many other easily crystal- 

 lizable salts. Furthermore Doelter 4 has succeeded in converting 

 many well-known amorphous precipitates into crystalline bodies 

 by means of stirring, pressure, impact, and high temperature. 

 Among the substances thus transformed are aluminium, chromium, 

 and iron hydroxids, and the sulphids of arsenic, antimony, 

 and zinc. 



With this much by way of introduction, we may now proceed to 

 a closer consideration of some of the better recognized properties of 

 colloid sols and gels. For convenience we shall first take up the 

 discussion of these systems from the more definitely physical point 

 of view, and later take up those properties which seem more defi- 

 nitely chemical. 



Physical Properties of Colloids. 1. FORM AND SIZE. Current 

 opinion seems to be leading rapidly to the general acceptance of the 

 hypothesis that in liquid systems of two or more components we have 

 to do with a continuous series of conditions ranging from coarse sus- 

 pensions through suspensions of increasing fineness (increasing de- 

 grees of dispersion) to finally the molecular and ionic states of solu- 

 tion. The opinion is also growing that, although for certain practical 

 purposes the classification of all such systems in one way or another, 

 as in terms of the various degrees of dispersion, may be useful, the 



3 V. Weimarn. Ibid., 1910, 7, 92, and "Grundziige der dispersoid 

 Chemie," 107-108. 



4 Ztschr. Chem. Ind. Koll., 1910, 7, 86. 



