76 Introductory Biology 



liquid, or gas) in which the particles exist in a colloidal condition are 

 known as phases. The colloidal particles are called the dispersed phase, 

 and the medium in which they are dispersed is called the dispersion 

 medium. Hence we have a variety of colloidal systems; some of the 

 more common are given in the accompanying table. 



Some of the properties and many important reactions of matter which 

 is in a colloidal state depend upon the great surface displayed by the 

 enormous numbers of minute colloidal particles which constitute that 

 particular matter. Possibly the great amount of surface exposed by small 

 particles may be illustrated as follows: A cube of matter having edges 

 one centimeter long has an exposed surface of 6 sq. cm, (six surfaces, 

 each 1 sq. cm. in area). If this cube of matter were divided into simi- 

 lar, smaller cubes, each having edges only 0.01 cm. long, the total num- 

 ber of small cubes would be 1,000,000. Each small cube has a surface 

 area of 0.0006 sq. cm., and the total surface area of all the small cubes 

 will be 600 sq. cm., or an area one hundred times greater than the origi- 

 nal large cube. However, if the original large cube were divided into 

 extremely minute cubes, each with a size of an average colloidal particle 

 (0.000,001 cm. diameter), there would result one million billion cubes 

 (each having edges 0.000,001 cm. long), and the total surface areas of 

 all the colloidal-sized cubes would be 6,000,000 sq. cm., or one million 

 times as great as the original cube. These 6,000,000 sq. cm. are the 

 equivalent of over 6,500 square feet, or a city lot 65 by 100 feet. It 

 should be recalled that the original cube was only 1 cm. square; however, 

 there is an enormous surface exposure when even a small block of matter 

 is properly divided into particles of colloidal size. 



Protoplasm may exist as a liquid sol (L. solvo, melt) which flows or 

 as a more solid gel (L. gelu, solid). Under certain conditions it may 

 change from the sol to the gel state, or from the gel to the sol, or back 

 again, depending on the relative distribution of the contained colloidal 

 particles. If particles are more or less uniformly distributed in a liquid 

 medium, the liquid flows easily (sol state), but if the particles are ar- 

 ranged in a network which contains the liquid, it would not flow (semi- 



