100 THE CHEMISTKY OF CREATION. 



as water. The nitrogen and hydrogen combine 

 to form ammonia in small quantities. The 

 carbon unites with oxygen and forms carbonic 

 acid, which is slowly given off. These processes 

 continue, until the form and substance of the 

 wood are no longer recognisable ; and the re- 

 mainder is now called humus. Its chemical 

 composition, although very variable, consists, in 

 general terms, of carbon, with a little oxygen 

 and hydrogen, and some insoluble earthy mat- 

 ters, formerly entering into the composition of 

 vegetable tissue. The carbonic acid given off 

 in this process of decay, either escapes into the 

 air, or, dissolved by water, it supplies the roots of 

 plants with a small portion of their food. Some- 

 times, when air is excluded, or partly so, as in 

 the centre of a hay-stack, chemical changes go 

 on so rapidly as to produce sufficient heat to set 

 fire to the stack, if the hay has been stacked in 

 too moist a state. Hence means are employed 

 to ventilate the interior of ricks by boring 

 large holes into them. Heaps of tow, hemp, 

 cotton, &c., take fire spontaneously in a similar 

 manner. 



The most familiar example we could select 

 of a decay of woody fibre, is the rotting of straw, 

 and the formation of vegetable manure. Those 

 to whom the busy farm-yard has at any time 

 been an interesting scene, will readily be able 



