140 LIGNIN. 



is to say, to ascertain of what it is composed, we find 

 that the greater part of it consists of carbon, oxygen, 

 hydrogen, and a small portion of nitrogen, combined 

 together. When we burn it, or in any other way 

 weaken the affinity which the elements have for each 

 other, they separate, and, by combining together, 

 generally form water, carbonic acid gas, and ammonia. 



318. When a plant is boiled in water, it is found 

 that part of the plant dissolves in the water, whilst 

 part remains insoluble, and we are unable by long- 

 continued boiling to make the whole of it dissolve in 

 the water. These, then, are two great divisions of 

 vegetable matter — that which is soluble in water, and 

 that which is not. By very simple operations of this 

 kind it is easy to discover that plants are composed 

 of a variety of different compound substances, readily 

 distinguished from each other by the different pro- 

 perties which they possess. Of those w^iich are 

 usually found in all plants, the most abundant are 

 called lignin, starch, gum, sugar, gluten, and albumen. 

 The four former consist of carbon, oxygen, and hy- 

 drogen alone, whilst the two latter contain, in addi- 

 tion to these elements, a portion of nitrogen. 



319. Lignin, or pure woody fibre, exists in almost 

 all plants; it constitutes the greater part of the stem, 

 wood, bark, and branches of trees; and is present, 

 though in smaller quantity, in the leaves and flowers 

 of trees, shrubs, and succulent plants. It is the most 

 solid constituent of plants, giving strength to those 

 parts in which any quantity of it exists. It may 



