CHEMICAL WORK OF ANIMALS. 457 



the most important is uric acid. All over the world, above ground and 

 below ground, excreta of all kinds are being added to the soil, or intro- 

 duced by the burrowing animals below its surface, and the chemical effect 

 upon the inorganic material of the soil can not but be very great. 



Man has promoted chemical work in various ways. It has been 

 pointed out that cultivation exposes the soil to the transportational agencies, 

 and this continually exposes new material to chemical agents. Crops are 

 planted and gTOwn. When the crop matures it is removed, and the soil is 

 again free from its active living plants. Thus there is continual alternation 

 between abundant and sparse plant life, and consequently conditions very 

 favorable for chemical reactions. Further, it is explained on pages 463-465 

 that man has oxidized great quantities of the organic material entombed in 

 the earth during past geological ag-es, and thus increased the amount of the 

 active chemical agent, carbon dioxide, in the atmosphere; and this of course 

 has accelerated to an unknown extent plant growth and chemical action. 



ANIMALS, DEAD, AND BACTERIA. 



Animals die both below and above ground. When dead their bodies 

 decompose and ultimately produce water, carbon dioxide, nitrates, some 

 free hydrogen and free nitrogen, and some sulphates. This process of 

 change to the ultimate products is accomplished by the bacteria, fungi, 

 oxygen, and water, precisely the same as in the plants. The process is 

 one of oxidation and is considered under that heading. (See pp. 461-466.) 

 As with the plants, the process concentrates carbon dioxide, nitrates, and 

 other compounds in the belt of weathering, thus placing important chemical 

 agents in a very favorable position to do active geological work. 



WORK OF SOLUTIONS. 



It has already been explained that in the belt of weathering there are 

 gaseous solutions, water solutions, and various mixtures of these which are 

 at work upon the rocks. The very important gases are oxygen and carbon 

 dioxide. It has been pointed out (pp. 76-81) that the activity of water 

 solutions is dependent upon (a) the compounds present, (b) the temperature, 

 and (c) the pressure. 



(a) The water solutions are likely to contain considerable amounts of 

 the salts formed by the union of the bases and acids which ordinarilv occur 



