FORMATION OF GUM. 155 



some other compound, which is itself in an active 

 state of decomposition. 



359. When strong sulphuric acid is poured over 

 lignin, it rapidly changes it into gum ; after a few 

 minutes the fibrous character of the lignin disappears, 

 and a thick slimy substance is formed ; if the acid is 

 then diluted with water, and chalk be then added to 

 neutralize the acid, sulphate of lime will be thrown 

 down, arid a solution of gum is obtained. If the acid 

 is left in contact with the gum it soon begins to char 

 it, and finally completely decomposes it. 



360. Starch may also be converted into gum by a 

 very simple process ; when starch is heated consider- 

 ably above the boiling point of water, it is changed 

 into a kind of gum ; large quantities of starch are thus 

 made into gum by roasting, and used by the calico- 

 printers under the name of British gum; by chemists 

 it is called dextrine. The same change may be ef- 

 fected by boiling gelatinous starch with dilute sul- 

 phuric acid ; it soon loses its gelatinous character, 

 ■becomes sticky, and is converted into gum. Starch 

 also undergoes this change when acted on by diastase 

 (685); a small quantity of this substance, added to a 

 considerable quantity of gelatinous starch, rapidly 

 converts it into gum. 



361. If, having converted a portion of lignin into 

 gum by the action of strong sulphuric acid, we add a 

 quantity of water 'so as to dilute the acid, and then 

 boil the whole together for some hours, the gum will 

 gradually be changed into sugar; by this process, then, 



