8 INTRODUCTION TO GENERAL PHYSIOLOGY 



Heterogeneous Systems and the Phenomena 

 at their Boundaries 



A glance at any living organism is sufficient to impress upon 

 us the fact that it is composed of a great variety of things that are 

 distinct from one another in space. In the amceba, for example, 

 the nucleus and the particles scattered about in the protoplasm do 

 not mix with the rest of the cell substance. Moreover, we have 

 seen that even the clear part is full of tiny particles. The individual 

 cells, as well as the whole organism of a higher plant or animal, 

 are what the chemist would call " heterogeneous systems," as con- 

 trasted with such systems as solutions of salts in water. If we take 

 a sample from any part of a solution of common salt in water, we 

 find it to have the same composition. It is a "homogeneous 

 system." It could be made heterogeneous, however, by the addition 

 of a solution of silver nitrate. The precipitate of silver chloride 

 could be separated from the liquid. If we divided up a living cell 

 into parts, these parts would not have the same composition. 



The various parts of a heterogeneous system the parts that do 

 not mix with one another are called phases. The name might 

 seem to imply that they have the same chemical composition, and 

 this is sometimes the case. Take, for example, ice floating on 

 water at the freezing point. They are separate phases, with the 

 same chemical composition. But this is not necessarily the case. 

 Charcoal, suspended in water, forms one of the phases of this two- 

 phase system. There are certain laws which control the behaviour 

 of heterogeneous systems, some of which we may briefly consider 

 here. Others will be met with later. 



Consider water in a basin. The molecules in the depth of the 

 water are exposed on all sides to the influence of molecules like 

 themselves, not only in chemical nature but in their state of 

 motion, etc. They are attracted equally in all directions. This 

 attraction, as we saw before, gives rise to the " internal pressure " 

 of the liquid. Those molecules at the surface, on the contrary, 

 are only exposed to the attraction of similar molecules on the 

 one side ; the other side is exposed to air, where the molecules 

 are very few in number, and not limited as regards their distance 

 from one another. There is, as a result, a continual force exerted 

 on the water molecules at the surface, trying to pull them down 

 into the liquid. This could not happen, of course, without 

 diminishing the volume of the water, and even then there would 

 always be molecules at the surface. But the molecules are so 

 close together in a liquid that they cannot be made to get closer 

 except by enormous pressure, or by decreasing their kinetic 

 migrations by cooling them. The result of the pull inwards can 



