CHAPTER VII 



COLLOIDS 



Substances which can be obtained in the crystalline state and which, 

 when in solution, are capable of readily diffusing through membranes, 

 are designated as crystalloids, and are to be distinguished from another, 

 larger group of substances not having these characteristics or having 

 them only in very minor degree the colloids. In every field of chem- 

 istry the properties of colloids have been studied extensively during 

 recent years, but in no field more than in that which covers the chem- 

 istry of biological fluids and tissues, into whose composition colloids 

 enter much more extensively than crystalloids. The subject of colloidal 

 chemistry has indeed become so extensive that an attempt to do more 

 than indicate some of the most important characteristics of colloids 

 would take us far beyond the limitations of this book. The far-reaching 

 applications of the subject in physiology and medicine are only begin- 

 ning to be realized. 



The term "colloid," or "colloidal," does not refer to a class of chemical 

 substances, but rather to a state of matter which is quite independent 

 of the chemical composition of the substance. We are familiar with 

 more colloids in the organic than in the inorganic world, yet they are 

 plentiful in both, and the same substance may at one time be colloidal 

 and at another noncolloidal. Indeed, under appropriate conditions prob- 

 ably all substances may assume the colloidal state not solids and liq- 

 uids alone, but gases as well. It is mainly with liquids, however, that 

 we are concerned in biochemistry. 



CHARACTERISTIC PROPERTIES 



The distinction between molecular* and colloidal solutions is a rela- 

 tive one. Suppose, for example, that we take a piece of gold in water 

 and divide it up into smaller and smaller parts. At a certain stage, the 

 particles will be so fine that they will remain in suspension and be in- 

 visible by ordinary means. They are then said to be in the colloidal 

 state. If we divide them further until they become molecules of gold, 

 a molecular solution will be obtained. In the colloidal state, there are 



*Molecular solutions include those of nonelectrolytes, such as sugar, and electrolytes, such as 

 inorganic salts. 



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