The Basis of Life 33 



Some compounds such as table salt dissociate nearly completely 

 while others dissociate only partially or not at all when dissolved. 

 In a true solution, the solute particles are of an extremely small size 

 and are equally distributed through the solvent. 



Suspensions. — A suspension is quite different from a solution 

 inasmuch as the particles are of a much larger size. The fine particles 

 of mud in river water are an example of a suspension. 



Colloids. — Intermediate in size between the particles of a solution 

 and a suspension are the colloidal particles. In some respects a colloid 

 resembles a true solution, for the colloidal particles are equally dispersed. 

 Each particle is considered to be 1/10,000 to 1/1,000.000 mm.* in 

 diameter. These particles are the dispersed plmse while the matrix in 

 which they occur is the continuous phase. As all the particles are of 

 the same electrical charge, they repel one another, and thus a true 

 colloidal solution never settles out. It is possible that large protein 

 molecules form these particles in living systems. 



Unlike a true solution, a colloid may occur as a somewhat rigid 

 gelatinous material. If water is lost from the continuous phase, the 

 system becomes more rigid and is know as a gel. When it picks up 

 water and becomes more liquid it is known as a sol. The ability to 

 change from gel to sol and back again is a property of utmost impor- 

 tance in the physiology of living organisms. 



Brownian Movement. — In 1828, Robert Brown, an Englishman, 

 was examining under his microscope some microscopic pollen grains 

 suspended in water. He noted that they moved in a random, vibratory 

 manner, tracing a strange zigzag pattern. Fascinated as he was, he was 

 unable to explain the cause of this movement. In fact, it took another 

 eighty years of observation for the real explanation to be developed. 



In 1908, a Frenchman, Perrin, began a series of experiments in 

 which he measured the behavior of various suspended particles. He 

 assumed that these movements were actually magnified movements of 

 molecules. The suspended particles, he reasoned, vibrated because 

 they were being continuously "bumped" by the molecules of the mediuni 

 in which they were suspended. His experiments showed the correct- 

 ness of his assumptions. In honor of Brown, this type of movement is 

 now called Brownicni movement. 



*1 mm. = 1/25.4 inches. 



