312 THE EQUILIBRIUM OF COLLOID AND 



densation, and breaking apart would readily take place if the osmotic 

 pressure or concentration of either constituent were reduced. 



In this way a salt which crystallises with water of crystallisation 

 has exact stoichiometric relationships with the water, and yet the 

 water and salt, which are both saturated compounds, can only be 

 held together here by residual affinities. 



To carry this short sketch of the controversy as to adsorption 

 versus chemical combination into detail would lead us far beyond 

 the limits of this chapter, so we may sum up with the statement that 

 between bodies of different chemical constitution there are varying 

 grades of affinity for union. At the one end of the scale there 

 are the typical chemical compounds, and at the other the more 

 physical unions 1 of a weaker type, and dependent upon the main- 

 tenance of certain appreciable pressures or concentrations of the 

 substances uniting, which have been called adsorptions ; but be- 

 tween these two there are all possible gradations, just as there are 

 all possible stages between crystalloids and colloids. 



Whether the theories and terminology of adsorption be accepted 

 or those of the formation of easily dissociable chemical combination, 

 or the middle view be taken that in some cases one occurs and 

 in other cases the other, the important experimental fact which 

 remains indisputable is that a type of union occurs which is only 

 stable so long as a certain pressure (concentration) is maintained, 

 and breaks up as the pressure diminishes, showing a range there- 

 fore at which association and dissociation of the union occur in 

 a fluctuating way accompanying variations in pressures within the 

 range. 



1 A great deal has been written as to the mode of physical union and how 

 it is brought about. It has been shown that any substance which lowers the 

 surface tension at a bounding surface or interface will tend to increase in con- 

 centration at that surface. In this way the formation of surface films of 

 protein and other colloidal solutions can be explained, similarly the formation 

 of a layer of dye on a fabric may meet with explanation, and a great many 

 if not all other cases of adsorption. But the question remains, Why does the 

 substance lower the surface tension ? and the view is still tenable that the 

 surface tension is lowered because of chemical affinity for the substance form- 

 ing the surface, or because the conditions on the surface favour chemical 

 condensation of the substance to form larger molecules or aggregates than in 

 the body of the solution. Also, an attraction of residual affinities may attach 

 the substance by adsorption, and after this anchorage true chemical union 

 may follow, 



