8 PHYSICAL CHEMISTRY 



of this viscous fluid. When two substances or fluids do not mix 

 homogeneously they are called phases. In the suspensoid solu- 

 tion one phase is the solid particles and the other the water. 

 In the emulsoid solution both phases contain water and colloid, 

 but one is very rich in water and the other is relatively poor in 

 water. Emulsoids may be precipitated with large quantities of 

 salts in the same way that alcohol may be salted out of water, 

 but they are not affected by the small amount of salt necessary 

 to precipitate suspensoids. The particles of typical emulsoids 

 cannot always be seen with the ultramicroscope, but the particles 

 of lecithin, a phospholipine, for instance, are usually distinct. 

 Many protein solutions which are typical emulsoids are trans- 

 formed into suspensoids by boiling. This change, which is called 

 denaturation, is caused by many substances, which are therefore 

 used in tests for proteins. Emulsoids increase the viscosity and 

 decrease the surface tension of water, whereas suspensoids do 

 not. Since all substances that reduce surface tension become 

 more concentrated in the surface film, the same is true of the 

 emulsoids. The emulsoid in the surface film gradually changes 

 into an elastic membrane, a so-called haptogen membrane. 



'Colloids have the peculiar property of forming jellies, called 

 gels to distinguish them from their solutions called sols. The 

 gel formed by a suspensoid consists of a spongelike structure 

 formed of rows of colloidal particles. No structure can be seen 

 in the gels of some emulsoids, but there is evidence for such a 

 structure in their elasticity. An emulsoid increases the viscosity 

 of water. The greater the concentration of the emulsoid the 

 greater the viscosity, until very concentrated solutions have the 

 viscosity of jelly, and are therefore gels. The viscosity of 

 emulsoid sols and gels is not entirely dependent on concentration 

 but is influenced by temperature and the presence of dissolved 

 substances. 



Whereas we may learn a great deal about the exchange be- 

 tween the cells and the exterior by studying the intake and out- 

 put of the lungs, the products of digestion that are absorbed by 

 the alimentary canal and the output of the kidneys, many cells 

 are so far removed from these organs that much is left to be 

 learned about their exchange. These cells are bathed by tissue 

 juice, which is probably somewhat similar to the blood plasma. 



