62 TEXTBOOK OF BOTANY 



molecules and has a definite surface which is electrically charged. In very 

 fine suspensions the particles may not settle out for months or years, 

 because of the constant bombardment of the water molecules and be- 

 cause the particles have similar electric charges. Suspensions differ from 

 solutions in that the water and the particles dispersed in it form a two- 

 phase system. The one phase is the solid particle, the other is the water, 

 and between each particle and the surrounding water is a definite 

 surface of contact. When a mass is broken up into fine particles, the 

 aggregate surface of the particles is enormous. For example, a one- 

 centimeter cube has a surface of 6 sq. cm., but when disintegrated into 

 particles of colloidal size the combined surfaces of all the resulting par- 

 ticles are equal to about 1.5 acres. 



Another tvpe of colloid is exemplified by milk, in which proteins and 

 fat-like compounds (lipoids) are dispersed in water which contains 

 other proteins, salts, and sugar in solution. Dispersed throughout this 

 liquid phase of milk are numerous visible globules composed largely of 

 fat. These globules slowly rise to the surface and form a layer of cream. 

 In butter the phases are reversed; fine droplets of water are dispersed in 

 fatty material. White of egg is a typical protein-in-water colloidal sys- 

 tem; the yolk is a colloidal system of many phases, with fats, proteins, 

 pigments, and other substances dispersed in water. When white of egg 

 is heated or treated with alcohol, vinegar, or strong salt solutions it 

 coagulates. When meat, which is largely protein, is boiled it also coagu- 

 lates. Gelatin desserts and fruit jellies are familiar examples of colloids. 



When a gram of gelatin is dispersed in 50 grams of hot water a highly 

 fluid colloid, called a sol, is formed. When it cools and stands for a few 

 hours it becomes a semi-solid jelly, or gel. The gelatin becomes finely 

 divided and dispersed in the water but does not dissolve. Its particles 

 are much larger than molecules. Owing to the elasticity and certain 

 other properties of this type of colloid, the dispersed particles are 

 thought to be fiber-like and arranged like twigs in a loose brush heap. 



When a gelatin-water gel is heated it becomes highly fluid, but after 

 it cools and stands for a time it again becomes organized into a more 

 or less rigid gel. Thus the gel and sol states of a colloid may be reversible, 

 and the change from the one to the other may be brought about by 

 changes in temperature, water content, acidity, or any one of several 

 other conditions. Drastic changes in the internal organization of a 

 colloid may result in coagulation. 



Protoplasm is a very complex colloidal system with protein, carbo- 



