CRYSTALLOID IN LIVING CELLS 13 



acidic groups, then we have a definite number of anchorages, and 

 when the saturation point of absorption is reached there must appear 

 stochiometric relationships although there has occurred no con- 

 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 stochiometric 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 article, 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 

 between 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 occurs in 

 a fluctuating way accompanying variations in pressures within the 

 range. 



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

 i 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 concentration 

 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 ? mid the view is still tenable that the surface tension is lowered because 

 of cli.-iuical affinity for the substance forming the surface, or because the conditions 

 on the surface favour chemical condensation of the substance to form larger mole- 

 cules or aggregates than in the body of the solution. Also, an attraction of residual 

 affinities may attach the sultstance by adsorption, and after this anchorage true 

 chemical union may follow. 



