ARTICLES 235 



any substance in the colloid state. This being so, we are 

 justified in comparing it with other states of matter. Tradi- 

 tionally it has been contrasted with the crystalline form, 

 but Von Weimarn * has recently shown the probability of 

 some colloidal salts having a crystalline structure. It would, 

 however, be unjustifiable to apply his results generally, even 

 to suspensoids — that is, to those systems in which the disperse 

 phase before dispersion is solid. The question also arises as 

 to what we are to understand by " crystalline," when the 

 surface energy is as large as that certainly present in colloid 

 systems. One of the most essential properties of the latter 

 is, in fact, the relative magnitude of surface energy as com- 

 pared with internal energy. As an immediate effect of this 

 large surface energy, we might expect adsorption phenomena 

 to play a large part in the chemistry of colloids, and recent 

 work has tended largely to confirm this view. Adsorption 

 effects are, however, modified to a certain extent by other forces 

 acting upon the colloid particle, which we will shortly consider. 

 Dispersion, — Before proceeding, it is of interest to consider 

 the methods of dispersion, or actual passage from the homo- 

 geneous, or coarsely heterogeneous, state of matter to the 

 more finely divided state which exists in all colloid systems. 

 It is convenient to consider the disperse phase in such systems 

 as being potentially a transitional stage between the homo- 

 geneous substance and a true solution. Viewed in this light, 

 colloidal solutions might theoretically be prepared in two 

 ways. We might either suspend the disperse phase in a suit- 

 able medium, and, by some means, increase the degree of 

 dispersion to the required extent ; or, starting with a true 

 solution, we might bring about aggregation of the molecules, 

 causing it to cease when it had reached a certain stage. The 

 actual methods of dispersion are, however, not so simple as 

 this, and vary in different cases. Some substances are met 

 with ordinarily in the colloid state, as, for example, gelatin 

 and albumin ; but with most substances we have to adopt 

 methods — chemical, electrical, or mechanical — suited to their 

 conversion to this state. In any case, it is necessary not only 

 to produce a system in which dispersion is of a certain order, 

 but also to prevent aggregation of the particles by some means. 

 Our knowledge of this second factor is very far from complete, 

 but stabilisation is usually ascribed to molecular forces at 

 the interface. We can recognise three classes of colloid solu- 

 tions, in which stability is due to adsorbed Uquid, to adsorbed 

 non-electrolyte, or to adsorbed ion." Of these three, more is 

 known of the last case than of the other two. 



^ Loc. cit., and Zur Lehre der Zustdnden der Materie, 19 14. 

 =» Bancroft, Second Brit. Ass. Rep, on Coll. Chem., 1919. 



