30 INTRODUCTION TO GENERAL PHYSIOLOGY 



The Colloidal State 



We are now in a' position to understand more about the particles 

 which were revealed in protoplasm by our special optical methods. 



Suppose that we imagine a small piece of gold immersed in 

 water, and by some means gradually divided up into smaller and 

 smaller fragments. Ultimately we shall arrive at the atoms, 

 beyond which we cannot proceed without altering the chemical 

 properties of the substance. But, before this state is reached, we 

 should find that the particles were small enough to be kept in 

 suspension by Brownian movement, and that the preparation would 

 show some new properties. It would appear clear but coloured, 

 and might be taken to be homogeneous unless a bright beam of 

 light were sent through it. When this is done the existence of fine 

 particles of gold is made manifest. Although such solutions of 

 gold, which are said to be " colloidal," could only be prepared with 

 great difficulty by simple mechanical disintegration, they can be 

 made easily by chemical decomposition of solutions of salts of gold. 

 The action of a reducing agent is to split up the salt, so that metallic 

 gold is obtained in a very finely-divided state (E., p. 176). 



Similar solutions can be made by appropriate treatment of 

 various substances usually regarded as insoluble. The gamboge, 

 already used, is one of these. So is the suspension of carbon 

 particles known as " Indian Ink." Such are called " suspensoids," 

 and consist of a solid phase suspended in a liquid phase. Since 

 the solid phase is completely surrounded by the liquid one, it is the 

 "internal phase," and may be compared to a number of islets 

 surrounded by the sea. But it is clear that the same constituents 

 might be arranged differently, similar to a number of small lakes 

 surrounded by land, such as might happen if the islands grew until 

 they touched one another. Here the solid phase would be external 

 and the water internal. The whole system would be solid, instead 

 of liquid. 



Further, the constituents of a colloidal solution may be two 

 liquids which do not mix with one another. These systems are 

 " emulsions," or, when their internal phase is very finely divided, 

 " emulsoids." A good example is cream, where the internal phase 

 consists of oil globules, the external phase is a watery solution. 

 When made into butter, a redistribution of phases occurs by the 

 oil globules uniting together ; the fat becomes external, the watery 

 solution in droplets surrounded by it. The meaning of the terms 

 sometimes used will be plain. The internal phase is the "dis- 

 persed " one, the external phase is the " continuous " one. It is 

 very likely that changes in distribution of phases plays an important 

 part in the mechanics of the cell and of its membrane. 



