DECAY OF VEGETABLE MATTER. 181 



one is taken by an atom of something else, and if the 

 process is allowed to go on, the whole may be converted 

 into, or rather replaced by, a mineral or metal, which forms 

 a model so exact that all the minute fibres and cells and 

 even the very texture of the original substance are clearly 

 distinguishable, though none of it remains. 



Pieces of petrified wood, in which the grain is distinctly 

 visible, are found in large quantities in the Suffolk Crag, but 

 petrifaction or mineralisation is not the common fate of dead 

 vegetable matter. When left freely exposed to the air, it is 

 slowly oxidised or burnt up, the hydrocarbons, cellulose, 

 and starch of which, whatever its nature, it chiefly consists, 

 being converted into carbonic acid and water. 



The light-coloured fibres of the stems and leaves are 

 gradually converted into a brown or black powdery 

 substance, whose weight and bulk continually diminish as 

 more and more carbonic acid and water are produced ; and, 

 with a suitable temperature and full supply of air, the 

 process goes on without interruption, though more and more 

 slowly, until nearly all the carbon and hydrogen have 

 returned to the air whence they came. The nitrogen, too, 

 has been set free and gone back, partly as nitrogen, partly 

 in combination with hydrogen as ammonia. 



In the island of Trinidad, where the heat is perpetual 

 and rainfall large, all vegetable fibre decays so rapidly that, 

 as Mr. Kingsley has said, there is hardly a dead stick or 

 leaf to be seen even in the primaeval forest. An English 

 wood, if left to itself, would be cumbered with fallen trees, 

 and in North and South America there are forests, in the 

 temperate zones, which are piled ten or fifteen feet high 



