34 Cellulose 



products which we have seen to be obtainable from them, to 

 deal with those esters which are formed by direct synthesis with 

 acid radicals. It will become evident, as we proceed with the 

 description of the derivatives of cellulose, that it only yields, 

 with any facility, such as result from reaction of its OH groups 

 with mixed radicals. It will appear from the composition of 

 the resulting esters about to be described that the empirical 

 unit C 6 H 10 O 5 contains at least three OH groups ; of the 

 remaining O atoms one certainly is carbonyl oxygen, though, 

 as it exists in the cellulose complex, it manifests no ' outside ' 

 activity. It is brought into play, however, in a number of 

 the decompositions of cellulose, and those determined by the 

 non-oxidising acids are chiefly to be noted in regard to this 

 point. It will become evident also that the molecular changes 

 by which this group is set free are, in effect, decompositions, 

 in the sense of a breaking down of the complex. The function 

 of the remaining O atom is obviously a problem of moment 

 in regard to the question of the constitution of cellulose, and 

 it will appear that the solution of the problem presents con- 

 siderable difficulties. 



Cellulose Acetates. The acetylation of hydroxy-com- 

 pounds generally affords the simplest evidence as to their OH 

 groups, their number and disposition or arrangement, in the 

 molecule of the compound. 



In the cellulose group, however, the problem is complicated 

 (i) by the difficulties of preparation and purification of the 

 acetates. The solutions are highly colloidal, and the ordinary 

 criteria of purity of the products are wanting. 



(2) By the difficulties of analysis ; the direct ' saponification ' 

 numbers often varying considerably from those obtained by 

 distillation of the volatile acid (after saponification), and both 

 numbers often failing to agree with the results of ultimate 

 analysis. 



