758 LIGNIN. 



Its solution dissolves oxide of lead. Nitric acid converts man- 

 nite into oxalic and saccharic acids, and not into mucic acid. 



Mannite is also one of the products of the viscous fer- 

 mentation of cane and grape sugar, which will be afterwards 

 described. 



GUM. 



Its formula is C 12 H n O n ; it loses an atom of water at 266 

 (130 cent.), but is then essentially altered. Gum is a principle 

 of constant occurrence in the juices of plants, and exuding from 

 the bark of trees, collects into drops, which are distinguishable 

 from resin by being soluble in water and insoluble in alcohol. 

 All the varieties of gum may be referred to two species, of which 

 gum-arabic (the produce of the acacia vera), and gum-tragacanth 

 are the, types. The first is slowly soluble in cold water, the last 

 does not dissolve in water, but swells up into a mucilaginous 

 mass, which, when boiled, gradually acquires the appearance of 

 ordinary gum. The solution of gum, known as mucilage, is a 

 thick, adhesive, insipid liquid, from which the gum is thrown 

 down by alcohol. Gum is precipitated by sub-acetate of lead, 

 as a white mass, insoluble in water. It is destroyed by the 

 strong acids ; nitric acid converts it into mucic acid. 



LIGNIN. 



The formula of lignin, dried between 300 and 350 is C 12 H 8 O 8 

 (Prout). The basis of woody fibre is aptly so named. It 

 constitutes about 95 per cent of baked wood, and is the most 

 durable product of vegetation. Pure lignin is obtained by 

 treating the sawings of wood, paper, or the fibre of lint and 

 cotton, successively with ether, alcohol, water, a diluted acid, 

 and a diluted caustic alkali, to dissolve all the matters soluble 

 in these menstrua. Wood contains in its vessels the various 

 constituents of the sap, of which the colouring principle attaches 

 itself to the lignin on evaporation, by a chemical affinity, such 

 as we avail ourselves of in dyeing vegetable fibre. It has been 

 observed by Hartig that the pores of wood also contain a cer- 

 tain quantity of starch, in spherical grains of a grey colour, 

 of which from one-fourth to one-fifth of the weight of the wood 

 may be obtained by mechanical means. The lignin of wood consists 



