CARBON IN PLANTS.

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intended to be driven into tlie ground ; the coat ofcharcoal thus formed protects the wood from decay fora much longer period than if not charred. When,however, carbon is heated, its chemical affinity foroxygen is greatly increased, and it no longer appearsto be the unchangeable substance which it is whilstcold.

57. Carbon has been already stated to be a necessaryelement of plants, which though so various in form andcolour, are nevertheless composed of very few elements.They consist almost wholly of oxygen, hydrogen,nitrogen, and carbon, which by combining in differentproportions, form all the principal parts of both plantsand animals.

58. The greater number of vegetable substances con-sist wholly of oxygen, hydrogen, and carbon; whilstanimal matters mostly contain, in addition to thesethree elements, a quantity of nitrogen. When wood ischarred, its elements are separated from each other; thehydrogen and oxygen combine together and form water,whilst the carbon is left behind. When wood is burntin the open air, both its hydrogen and carbon com-bine with oxygen, causing flame; the combination ofcarbon with oxygen proceeds slowly and steadily, thecarbon continuing to glow until all is consumed;hydrogen, on the other hand, combines with oxygenmore rapidly, burning at once, with a flame.

59. Whenever a substance burns with a flame, wemay be sure that it contains hydrogen ; and whenever