CELLULOSE 205 



nection. At the outset it is important to distinguish clearly 

 between the irreversible pectin gels formed by the action 

 of pectase, and the reversible gels concerned in the formation 

 of fruit jellies or jams. As pointed out above, the former are 

 most probably composed of insoluble calcium pectate which 

 when once formed cannot be got into solution again. On the 

 other hand, it is known that soluble pectin forms in water not 

 a true solution but a sol which in about 2 per cent concentration 

 is fairly viscous but does not set to a gel ; in order to produce 

 a gel from such a solution it requires to be boiled in water 

 containing about 60 per cent of cane sugar and approximately 

 I per cent of tartaric acid ; on cooling the resulting mixture sets 

 to a gel which, according to Sucharipa, is due to the fact that the 

 pectin is insoluble in such a solution of cane sugar. Care must 

 be taken not to boil for too long as otherwise hydrolysis may set 

 in which will entail loss of methyl alcohol ; it appears from 

 the work of Nanji and Norman * and others that the jellying 

 power of a pectin is a function of its methoxyl content ; 

 the authors mentioned have worked out a micro-method for 

 the determination of methyl alcohol. 



CELLULOSE. 



The term cellulose should be taken in general to connote a 

 group of substances rather than a single chemical compound ; 

 used in this generic sense, it comprises a number of substances 

 of somewhat different origin and characters, whose chief 

 common properties are their physiological origin and their 

 function in forming the basis of the material which is isolated 

 by the protoplasm of the living cell for the purpose of form- 

 ing the wall or periphery of that cell. Though met with 

 chiefly in the vegetable kingdom, its occurrence in the animal 

 kingdom is not unknown, since a substance described as 

 tunicin, said to be identical with cellulose, has been found in 

 the cell walls of certain tunicates and insects. In the course 

 of time the cellulose originally formed is altered by the addi- 

 tion to it of various secondary products known as encrusting 



* Nanji and Norman : " J. Soc. Chem. Ind.," 1926, 45, 337 ; Baker : 

 " J. Ind. Eng. Chem.," 1926, 18, 89. 



