ORIGIN AND ACTIVITY OF AZO-HUMIC ACIDS. 219 



colored with organic matter. There would thus appear to be present an 

 abundance of the humic acids which are the intermediate products in the 

 decay of vegetable matter between the original vegetable tissues and the 

 final oxidation products, water and carbonic acid. 



Thenard* has shown that while the simple humic acids are compara- 

 tively inert, they readily absorb nitrogen from the air, being converted 

 into the azo-humic acids, the nitrogen not being present as ammonium, 

 although it forms ammonium on the further oxidation of these com- 

 pounds. The latter have a strong affinity for silica, combining with it 

 to form a new series of acids in which the amount of silica is apparently 

 proportional to the amount of nitrogen contained in the azo-acid. Thus 

 Thenard found that simple humic acid dissolves only 0.8 per cent of 

 silica, while azo-humic acid combines with from 7 to 24 per cent, depend- 

 ing on the content of nitrogen. The compounds thus formed are easily 

 soluble in the alkaline carbonates, forming alkaline salts of the silico-azo- 

 humic acids; but potassium carbonate must be abundant at all the 

 localities where the etching has been observed, being supplied to the 

 surface waters by the ashes resulting from the frequent burning of the 

 forest litter. 



Chemical Reactions involved in the Solution of Silica under 



atmospheric conditions. 



The principal steps in the complicated chemical process would appear 

 to be the following: By the oxidation of the vegetable tissues in the 

 process of decay the humic acids are formed, chiefly humic and crenic. 

 These absorb varying quantities of free nitrogen from the air forming 

 the azo-humic acids, which in turn combine with free silica. The result- 

 ing acids combine with alkaline carbonates, particularly potassium car- 

 bonate derived from ashes, to form easily soluble salts. It is quite 

 probable that the presence of the potassium carbonate is an essential 

 factor in the process. 



In most cases the etched surfaces of the pebbles described above sup- 

 port a more or less abundant growth of cryptogamic vegetation, which 

 in some cases forms a nearly continuous covering, and in others only 

 scattering and very minute patches. It is most abundant on the pebbles 

 in the bed of Clifty creek and least abundant on those at Nuttall, which 

 receive the drip from a cliff and are thus kept wet most of the time. 

 This vegetable covering might facilitate the solution of the pebbles in 

 two ways — first, by supplying humic acids directly from their own decay, 

 and, second, by absorbing solutions of those acids from other sources 



* P. Thenard : Compt. Rend., vol. lxx, 1870, p. 1412. 



