CHAPTER IV 



COLLOIDS 



As we have already seen, living organisms are formed essentially 

 of liquids. These liquids are solutions of crystallizable sub- 

 stances or crystalloids, and non-crvstalli/able substances or 

 colloids — a classification which we owe to Graham. 



The liquids are the most important constituents of a 

 living organism, since they are the seat of all the chemical 

 and physical phenomena of life. The junction of two liquids 

 of different concentration is the arena in which takes place 

 both the chemical transformation of matter and the correlative 

 transformation of energy. In a former chapter we have passed 

 in review the class of crystalloids, we will now turn our attention 

 to the characteristic properties of colloids. 



Colloids. — Colloids differ from crystalloids in that they 

 do not form crystals from solution, being completely 

 amorphous when in the solid state. The solution of a 

 colloid solidifies in the same form which it possessed in the 

 liquid state, the solvent being enclosed in the meshes of a sort 

 of network formed by the solute. This form is approximately 

 retained even after the water has evaporated by drying, the 

 passage from the liquid state of solution to the solid state being 

 effected through a series of intermediary states, such as a clot, 

 coagulum, or jelly. This passage from the state of solution 

 into a state of jelly is called coagulation. Some colloids, such 

 as gelatine, coagulate with cold; while others, such as egg- 

 albumin, coagulate with heat. Some, like the caseine of milk. 

 require the addition of certain chemical substances to set up 

 coagulation ; while still others, such as the fibrin of blood, appear 



to coagulate spontaneously. The physical phenomena of 



36 



