462 A TREATISE ON METAMORPHISM. 



Thenard) is the most important in the soil. Some of the laboratory prop- 

 erties of humic acid are as follows: It is amorphous; it decomposes at 

 145° C; after drying, it requires 13,784 parts of boiling water to dissolve 

 it; when undried, it requires 8,333 parts at 6° C. and 625 parts at 100° C. 

 to dissolve it. With humic acid in the soil are various other acids, such as 

 ulmic acid (C 18 H 14 6 or C 18 H ]6 7 , Berthelot and Andre), crenic acid 

 (C ]2 H 12 8 , Mulder) , and apocrenic acid (C 24 H 12 12 , Mulder)." 



The humic acids are first produced by the oxidation of the cellulose. 

 The other organic acids appear to be produced by further oxidation of 

 the humic acid. Finally, as the process of decomposition continues, 

 humic and the other organic acids by their decomposition are further 

 oxidized and broken up, so that the ultimate products are carbon dioxide 

 and water. 



At all stages in the process the acids are active. The humic acid and 

 other organic acids act upon the various inorganic compounds, especially 

 the silicates, forming salts. However, the acids of these salts are broken 

 down to carbonic acid. Also the breaking down of the free humic acid 

 and other organic acids produces carbon dioxide. Therefore the most 

 important result of the decomposition of the cellulose is the production of 

 carbon dioxide. By the process of plant decay carbon dioxide is thus 

 concentrated in large quantities in the belt of weathering — that is, in the 

 place where it can do its most active work. The process of carbonation 

 (explained on pp. 473-480) is consequently largely due to this concentration. 



Besides their direct chemical activity, humic and other organic acids have 

 various other important effects in the belt of weathering. As a result of the 

 dark color of humic acid it is an absorbent of the sun's heat, Moreover, it 

 has a higher specific heat than soil, and hence is able to retain a large 

 quantity of heat. Thus, in this way, the presence of humic acid promotes 

 chemical activity. Furthermore, humic acid is hydroscopic, and in sandy 

 soils greatly increases the holding power for water; and it has been noted 

 that the chemical activity in soils is very largely dependent upon the 

 amount of water they hold. Finally, humus in the soils holds ammonia, 

 nitrites, nitrates, and soluble sodium and potassium conqjounds, in other 

 words, plant foods. These plant foods may be held in some cases as 



"See also Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 

 1897, pp. 189-190. 



