92 PROTOPLASM 



suspensions of solid particles are in other respects so typically 

 colloidal that to exclude them from the colloidal state would be 

 arbitrary and make for confusion. Yet we must agree with 

 Duclaux in so far as to recognize two main and rather distinct 

 groups of colloids, the suspensions and the gluelike substances. 

 Among the characteristics distinguishing these two groups of 

 colloids is the capacity of the gluelike substances to form gels, 

 or jellies. (Certain of the metal (oxide) suspensions form gels 

 and thus constitute a connecting link between the two groups.) 



Objection to the term "colloid" has been voiced. The word 

 is unfortunate, as are all words ending in -oid. To say that 

 a substance is "like" something else is to admit ignorance of it 

 except in certain particulars. But this is, after all, the way that 

 all descriptions begin. The classical chemist disapproves of 

 grouping substances so diverse as are gelatin (a protein), cellulose 

 (a carbohydrate), charcoal (carbon), silica (sand), and colloidal 

 gold (a metal) all under the one term colloid. But the classical 

 chemist thinks in terms of chemistry, while the colloidal chemist 

 thinks in terms of physics. All of the above enumerated sub- 

 stances, when in the colloidal (finely dispersed) state, have certain, 

 physical properties in common — they do not pass through a 

 parchment-paper membrane, and they scatter light. Colloidal 

 chemistry does not have to do with different kinds of matter 

 but with matter in a characteristic state. Gold is still a metal, 

 benzene a hydrocarbon, and albumin a protein when in the 

 colloidal state. Colloids are different kinds of substances occur- 

 ring in a similar and unique condition. It might, therefore, be 

 well to avoid the word colloid and to speak instead of the 

 "colloidal state" or of "colloidal systems." As such systems 

 stand apart from others owing to their physical rather than their 

 chemical properties, the subject dealing with them could better 

 be termed "colloidal physics" rather than "colloidal chemistry." 



Methods of Preparation. — Colloidal suspensions may be made 

 by one of two general methods — dispersion or aggregation. Large 

 masses may be dispersed or broken up into minute particles, or 

 molecules may be brought together into aggregates of colloidal 

 size. There are a number of ways of accomplishing both of these 

 processes. Coarse matter may be dispersed mechanically, 

 electrically, or chemically. A solid substance may be ground into 

 a very fine powder which is shaken in water. Some of the par- 



