108 Cellulose 



titrated. A portion is drawn off and tested for nitric and 

 nitrous acids. If free from these acids, the titration number 

 may be taken as representing the acetic acid. Otherwise a 

 fraction of the distillate must be further treated for the elimi- 

 nation of the nitrogen acids. For this purpose it is acidified 

 with sulphuric acid, and digested for some hours with a ' copper- 

 zinc couple.' The solution is then poured off, and distilled, as 

 before described, from a slight excess of sulphuric acid. 



These determinations of acetic acid are destined to con- 

 tribute, in an important way, to the solution of constitutional 

 problems, and the student should master the details of th2 

 somewhat laborious process above described. 



In certain cases, other volatile acids may be formed. It 

 is advisable to control the results by testing the distillate 

 qualitatively, and should there be indications of the presence 

 of other acids, e.g. formic acid, a fraction should be redistilled 

 from chromic acid, and the distillate again titrated. 



The distillates also may be divided, a portion being titrated, 

 and the acid in a portion converted into silver salt in which 

 the silver is determined. 



Having thus described in general terms the constituent 

 groups of the typical lignocelluloses, and more specially the 

 methods by which they may be quantitatively estimated, 

 directly or indirectly, it is necessary to point out that so far 

 nothing has been said as to the mode of union of these groups 

 in the fibre-substance. The evidence on this side of the subject 

 will be given in due course. It is sufficient, for the present, to 

 remember that we are dealing, on an empirical basis, with the 

 well-ascertained chemical constants of lignification constant 

 for any given lignocellulose, but varying considerably from 

 member to member of this wide and varied group of plant 

 constituents. It may not be out of place also to insert at this 

 point a caution against the possible inference that the above 



