CHAPTER II. 

 SURFACES. 



WE have seen in the preceding chapter that both colloidal solutions 

 and jellies are to be regarded as two-phase systems. In dispersed 

 systems, the surfaces of contact acquire an overpowering importance 

 by reason of their enormous development. 



In order to get an idea of the increased development of surface 

 attending progressive subdivision, I give below a table taken from 

 WOLFGANG OSTWALD: 



From this, we can understand how small surface forces may, in a 

 dispersed system, become most important, and mask other phe- 

 nomena. We shall proceed to the study of the properties of sur- 

 faces in the following pages. 



If we compare a point A (Fig. 1) in the interior of a phase, e.g., 

 a fluid, with one on the surface A', e.g., in contact with air, we notice 

 that the former is surrounded on all sides by an impervious mass, 

 whereas the latter, surrounded on only one side by such a mass, 

 experiences an attraction to the fluid phase. The pressure with 

 which the surface layer is drawn inwards is called inward attraction 

 (binnendruck) . 



If we imagine a drop of water to be on a surface which it does not 

 moisten, e.g., a leaf, there is a pulling towards the center from all 

 sides, which means that the drop takes a spherical form, in order 

 that it may have the smallest possible surface. The surface acts 



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