SECTION VIII. 

 THE COLLOIDAL STATE. 



A KNOWLEDGE of the properties associated with the colloidal 

 state of matter is of the greatest importance in the study of 

 the chemical and physical problems presented by both plants 

 and animals; for this reason some of the more important 

 facts concerning colloids are here set forth. To illustrate 

 the bearing of this subject on plant chemistry, it is only neces- 

 sary to point out that the protoplasmic contents of any living 

 cell exhibit many of the properties of colloidal solutions, and, 

 indeed, it is held by some that the chief vital function of proto- 

 plasm is due to its acting as a colloidal medium. 



Apart, however, from the living cell contents, many of the 

 reserve and waste products of the vital activity of the cell are 

 colloidal substances. Thus, for example, the cell wall itself 

 is composed of cellulose, a substance which exhibits all the 

 characteristic properties of colloids, while starch, resins, gums, 

 rubber, proteins, and enzymes are all colloidal in nature. More- 

 over* many of the 'processes of dyeing and staining employed 

 in microscopical technique are directly due to the colloidal 

 nature both of the material to be stained and of the staining 

 solution ; further, a number of the properties of soil and humus 

 are directly attributable to the colloidal properties of these 

 substances. 



Before considering the properties of matter in the colloidal' 

 state, it is necessary to explain the origin of the term colloid. 

 Wnile studying the laws of diffusion in liquids, Thomas 

 Graham found that water soluble substances could be divided 

 into two classes : 



(a) Those that diffused relatively quickly, and 



(b) Those whose rate of diffusion was very slow or im- 



perceptible. 



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