22 



THE FOUNDATION 



Part I 



oysters), natural opal, water in silicates; (5) liquid in a liquid — gelatin in 

 water (gelatin may be a liquid or solid); (6) liquid in a gas — fog. Fog and 

 mist are actually solid particles in gas since the water molecules are gathered 

 on solid particles. It has been noted that at 6 a.m. the air over London may 

 be clear and at 9 a.m. there may be a dense fog. The onset of the fog is 

 largely due to the smoke that has provided particles on which the water 

 gathers. 



The most important of all mixed substances is protoplasm. It is a colloid, 

 the most complicated, most studied, and still largely unknown one without 

 which life does not occur. This colloid varies in consistency; when it thickens 

 its droplets swell, come closer together, and become a gel; when it thins, the 

 droplets do not absorb water, are smaller and farther apart, and form a sol 

 (Fig. 2.11). Protoplasm is a reversible colloid that may change from sol to 

 gel and return. Such changes may be seen through the microscope in any 

 ameba. White of egg is a gel when heated but it will not return to a sol. 



Emulsions. Although containing larger droplets than most colloids, emul- 

 sions are similar to them. Familiar emulsions are whole milk, egg yolk, and 

 mayonnaise dressing. 



Diffusion and Osmosis 



Diffusion is the movement of a gas or liquid from points of greater to those 

 of lesser concentration continued until an even distribution is achieved 

 throughout the available space (Fig. 2.12). Mice find the cheese from the 



D 



Water 



Sugar 



Diffusion 



Osmosis 



Permeability 



Fig. 2.12. Diagrams illustrating diffusion. In simple diffusion (A and B), mole- 

 cules of sugar without any barrier become evenly distributed among the molecules 

 of water in consequence of the motion of both. In osmosis, the diffusion through a 

 semipermeable membrane (C and D), the molecules of water can pass through the 

 membrane in either direction. They continue to do so until their number is equal on 

 each side of the membrane. Thus, the level of the sugar solution is raised. The mole- 

 cules of sugar, imprisoned by their larger size, continue to hit against the membrane 

 in their random movements exerting the force called osmotic pressure. In the com- 

 plete permeability (E) both kinds of molecules pass through the membrane at the 

 same rate and the solutions have uniform content on each side. 



