The Typical Cellulose and the Cellulose Group 15 



The most effective reagent is prepared as follows : zinc is 

 dissolved to saturation in hydrochloric acid and the solution 

 evaporated to 2-0 sp.gr. ; to 90 parts of this solution are 

 added 6 parts potassium iodide dissolved in 10 parts water, 

 and in this solution iodine is dissolved to saturation. By this 

 reagent cellulose is coloured instantly a deep blue or violet. 



A superficial examination usually suffices to identify cellulose in 

 the mass, and an examination with the microscope establishes the 

 histological characteristics of the substance. There are cases, 

 however, in which distinctive tests require to be applied, and these 

 will be selected in order of convenience. Thus, by means of the 

 chemical tests, cellulose has been identified as a constituent of 

 many animal tissues (see p. 87) ; in these cases, of course, it could 

 be identified in no other way. 



It will be seen as we proceed that a number of the properties 

 of cellulose are common to many of the * compound celluloses ' 

 which are widely distributed in the plant world ; these are, how- 

 ever, differentiated by the special reactions depending upon the 

 compounds or groups with which the cellulose may be com- 

 bined. 



Lastly, the cellulose group proper includes a number of sub- 

 stances which are differentiated from the typical cotton cellulose 

 in some specific property. These will be noted subsequently. 



Compounds of Cellulose. The chemical inertness of 

 cellulose is a matter of every-day experience in the laboratory, 

 where it fulfils the important function of a filtering medium in 

 the greater number of separations of solids from liquids. The 

 functions which it discharges in the plant world as well as the 

 numberless uses which it subserves in the world of humanity 

 are all referable to the predominance of these negative 

 characteristics. Cellulose, however, is a poly-hydroxy- com- 

 pound, and enters into a number of reactions characteristic of 

 the alcohols. These reactions, and the products of synthesis 

 resulting from them, we shall deal with in order, proceeding 

 from the less to the more definite. 



