60 CARBON IN PLANTS. 



90. Under common circumstances, carbon is one of 

 the most unchangeable things we know ; neither air, 

 water, nor any of the substances commonly found in 

 nature, have any action upon it ; and hence the prac- 

 tice of charring the lower parts of wooden posts, which 

 are intended to be driven into the ground ; the coat 

 of charcoal thus formed, protects the wood from decay 

 for a much longer period than if not charred (882). 

 When, however, carbon is heated, its chemical afl5- 

 nity for oxygen is greatly increased, and it no longer 

 appears to be the unchangeable substance which it is 

 whilst cold. 



91. Carbon has been already stated to be a neces- 

 sary element of plants, which, though so various in 

 form and color, are nevertheless composed of very 

 few elements. They consist almost wholly of oxygen, 

 hydrogen, nitrogen, and carbon, which, by combining 

 in different proportions, form all the principal parts 

 of both plants and animals. 



92. The greater number of vegetable substances 

 consist whollyof oxygen, hydrogen, and carbon; whilst 

 animal matters mostly contain, in addition to these 

 three elements, a quantity of nitrogen: some of the 

 compounds of plants, however, resemble animal mat- 

 ter in containing nitrogen. When wood is charred, 

 or decomposed by heat, its elements are separated 

 from each other; the hydrogen and oxygen combine 

 together and form water, whilst the carbon is left 

 behind. When wood is burnt in the open air both 

 its hydrogen and carbon combine with oxygen, caus- 



