COLLOIDS. 15 



these two conditions is often very indefinite. Certain chemically definable 

 classes of substances, such as proteins, occur only or chiefly in the col- 

 loidal condition while others, such as the inorganic salts, occur as crys- 

 talloids. Finally we find others that can occur in both forms, namely 

 the soaps (page 17). In short the difference between the crystalloid and 

 colloidal condition may be considered in that the crystalloids occur in 

 solution as molecules of medium size while the colloids are either very large 

 molecules, molecular aggregations or at least particles of a larger spacial 

 volume than the crystalloids. According to such a conception many 

 properties of the colloids can be explained. 



In order to give a better review we will give a classification of the 

 colloids which seems, for the present, to be rather universally accepted. 

 This was first suggested by PERRiN 1 and later accepted by HoBER, 2 

 A. MiJLLER, 3 and Wo. OsTWALD, 4 although different authors use different 

 names for the two classes. The classifications of HARDY 5 and ZSIG- 

 MONDY 6 have also much in common with the classification given below. 



One of the two groups of colloids is called hydrophile colloids (emul- 

 sion colloids, emulsoides) because in the aqueous solution a certain rela- 

 tion still exists between the dissolved substance and the solvent which 

 is evident especially by a certain viscosity of the solution. The hydro- 

 phile colloids often gelatinize on cooling, the gel is again soluble in 

 water (reversible), and in general the hydrophile colloids are separated 

 from their solution by electrolytes with greater difficulty than the col- 

 loids of the second group. Bodies of the greatest importance for phys- 

 iological chemistry like the proteins, starch, giycogen, and soaps in 

 watery solution belong to the hydrophile colloids. 



Contrary to the hydrophile colloids, the colloids of the colloidal metal 

 type are called suspension colloids (suspensoids) as they must be con- 

 sidered as suspended solid particles in a solvent and have no close 

 relation to the solvent. The viscosity of the solution does not differ 

 much from that of the pure solvent; besides this, the suspension col- 

 loids do not gelatinize, do not swell up, and are readily precipitated 

 by electrolytes. To this group belong the metallic sols, the colloidal 

 metallic sulphides, and certain typical suspensions obtained by dissolving 

 water-insoluble substances in another liquid (alcohol, acetone) and then 

 pouring this solution into a large volume of water. In this way the 

 substance is precipitated in a finely divided condition. Such suspensions 



1 Journ. de Chimie phy., 3, 84 (1905). 



2 Physik. Chem. d. Zelle u. Gewebe, 2 Aufl. (1906), 208. 



3 Allg. Chemie d. Kolloide (1907), 187. 



* Zeitschr. f. Chem. u. Ind. d. Koll., 1, 331 (1907), 



5 Proc. Roy. Soc., 66, 95 (1899). 



6 Zur Erkenntnis d. Koll. (1905), 16. 



