306 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



Important as is the carbonic acid brought down in rains in Brazil, the 

 great quantity produced on the ground, esi)ecially in the forest-covered 

 portions, is vastly more so. It comes from the decay of organisms upon 

 the surface and in the ground and from the breathing of subterranean 

 animals. This has been considered under the head of " Organic chem- 

 ical agencies." 



Nitrid acid (HNO.^). — Little account has been taken of the amount and 

 influence of the nitric acid brought to the earth in rains and of the part 

 it performs in the decomposition of rocks in the tropics.* Nitric acid 

 is produced in the air by electric discharges, and such discharges are 

 much more violent and more frequent in the tropics than in temperate 

 regions, and it may reasonabl}^ be expected therefore that the amount of 

 nitric acid thus produced is proportionately larger.f That such is the 

 case has also been determined by actual examinations of rain-waters in 

 temperate and tropical regions. It is true that the composition of the 

 granites and gneisses is such that these rocks are not attacked directly 

 by nitric acid, but, as is well known, time and the complex processes of 

 weathering eventually bring them witliin the range of influences by 

 which they are at first but little affected. Falling upon a soil covered 

 with organic matter, there can be no doubt but that this nitric acid is 

 quickly reduced to ammonia; but, as has been suggested by Julien, it 

 would tend even in this case to form organic acids of higher oxida- 

 tion.]: 



Muntz and Marcano have determined from two years' observations, 

 embracing 121 rainfalls in 1883-'84-'85, the amount of nitric acid con- 

 tained in the rain-water at Caracas, Venezuela. § These are the only 

 determinations made in the tropical i)art of South America of which I 

 have any knowledge. The climatic conditions at Caracas, Venezuela, 

 seem to make the observations at that point readily available for Brazil, 

 for Muntz and ^larcano remark that " the climate is characterized by a 

 temperature that varies but little, by the unequal distribution of the rains 

 and by the violence and frequenc}'' of storms."' The determinations from 

 19 rainfalls at Saint Denis, Bourbon island, are especially interesting, as 



*So far as I know, the first suggestion of the influence of nitric acid in rock decomposition in 

 the tropics was made by Heusser and Claraz in their article upon the Gisement et exploitation du 

 diamant dans la province Minas Geraes au Bresil. Aunales des Mines, 5me s6r , xvii, 1860, p. 291; 

 also Zeitschrift der Deutchen Geol. Gesell., xi, p. 448. 



flMuntz and Aubin say that carbonic acid is also produced by electric discharges (Comptes 

 Rendus, 99, 1884, pp. 871-874), but the amount of carbonic acid brought down by rain was accepted 

 from the determinations regardless of the method of its formation. It is possible, however, that 

 the electric discharges make a perceptible difference in the carbonic acid, and that the amount 

 thus produced in the tropics will necessarily be greater than in temperate regions. 



X Proceedings Amer. Assoc. Adv. Sci., 1879, p. 327. Ammonia salts also hasten mineral solution. 

 Johnson's How Crops Feed, pp. 139-140. 



§ Comptes Rendus, 108, 1889, pp. 1062-1064. 



