12 FUNDAMENTAL PRINCIPLES 



with the two sides of such a membrane, the hquids tend to mingle by the 

 passage of molecules or atoms through the openings. When one or both 

 of these two hquids contain sohds, other hquids, or gases in solution, the 

 particles of these substances also may pass through the same "^openings. 

 By the use of these membranes we can distinguish two categories of 

 substances. Those substances which when they are in solution are 

 capable of passing through such a membrane are said to be crystalloidal 

 and if they normally exhibit this character ar- often called crystalloids, 

 while those which will not pass through aie cermed colloidal kncl called 

 colloids. But many crystalloids may be made to assume a colloidal 

 condition. Of course it is a matter of relative size of particles and 

 openings, but in general it is true that crystalloids are substances which 

 exist in molecules of very small size or as atoms in a solution, while 

 colloids are substances which exist as particles of larger size, are dispersed 

 in hquids in the form of suspensions, and do not form true solutions. 

 These colloidal suspensions are thicker or more hke glue than are crystal- 

 loidal solutions. From crystahoidal solutions the substances are easily 

 obtained in crystalline form, but this is not true of colloidal suspensions. 

 Oils and fats and proteins, such as the albumen which forms the white of 

 eggs, are colloidal. This separation of cohoidal from crystalloidal sub- 

 stances is known as dialysis and the membranes which effect the separa- 

 tion, as dialyzing membranes, or dialyzers. 



Other membranes are found in the body which under similar condi- 

 tions permit some liquids or gases to pass through them and prevent 

 others from so doing, but the passage takes place as a result of solution 

 in the membrane and not through openings in it. Such a membrane is 

 termed a semipermeaUe membrane. This process is known as osmosis. 

 In this case the force behind the movement is called osmotic pressure. 

 Osmotic pressure is subject to the same laws as is the pressure of a gas 

 on the wahs of the container in which it is and against which it exerts 

 pressure because of its tendency to expand. 



Whenever two hquids of different densities are in contact and do not 

 mix, the plane of separation, which is in effect a surface film, acts hke a 

 semipermeable membrane; some substances will pass through it, others 



will not. 



19. Colloidal Emulsions.— In case the suspended droplets m an 

 emulsion are colloidal, the mixture is termed a colloidal emulsion. The 

 liquid in suspension is dispersed and is called the disperse phase, while 

 the other hquid is called the dispersion medium, or the continuous phase. 



A colloidal emulsion is more or less jelly-like. It may at one time 

 become thinner and assume the condition of a sol or at another time 

 become thicker and assume the condition of a gel. This may be due to 

 the transfer of liquid from the disperse phase into the dispersion medium, 

 or vice versa, without the addition of more liquid from without. The 



