708 STUDIES FOR STUDENTS 



oxygen, with minute quantities of carbonic acid (from 2.5 to 3.5 

 parts in 10,000), and in the vicinity of large cities and in volcanic 

 regions, of appreciable quantities of sulphuric and hydrochloric 

 acids as well. With rare exceptions these last may be consid- 

 ered as existing not as free acids, but in combination as sulphates, 

 and chlorides. This, in addition to their limited distribution 

 justifies us in largely ignoring them in the present discussion. 



Nitric acid, nitrogen a?id ammofda. — Much has from time to 

 time been written regarding the occurrence of nitric acid in the 

 atmosphere, and its supposed significance in relation to the sub- 

 ject under discussion. It seems now, however, to be the 

 generally accepted opinion among chemists, th.?i\. free nitric acid 

 in the atmosphere is a thing of comparative rare occurrence and 

 if occurring at all, is present only in very minute quantities. 

 The researches of Boussingault, Cloez, De Luca and others did, 

 it is true, indicate the presence of the acid, but as ammonia is 

 also almost invariably present in amounts sufficient or even in 

 excess of that needed to combine with it as a nitrate, the con- 

 clusion seems unavoidable that in the large majority of cases the 

 presence of free nitric acid is impossible, or it exists only 

 momentarily during times of great electrical disturbance (as 

 during thunder showers). As nitrate of ammonia its presence is 

 almost universal. It is probable that neither these gases nor their 

 salts have any direct influence in promoting rock decomposition. 

 It has been demonstrated, however, that nitrogen compounds 

 and nitrogenous matter in the soil, may become subject to 

 nitrification through the action of bacteria, whereby ammonia, 

 nitrous or nitric acid, carbon dioxide and water are formed, 

 though as Wiley says, "The ammonia and nitrous acid may not 

 appear in the soils, as the nitric organism attacks the latter at 

 once and converts it into nitric acid."^ (See further under 

 Influence of Plant and Animal Life.) 



In considering the efificacy of these agents as rock destroyers 

 we must not lose sight of the fact that the supply of nitrogen 

 in the soils is as a rule far too small to supply the demands of 



' Wiley, Principles and Practice of Agricultural Analysis. 



II 



