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



great depth's, they can do so only when the way is prepared for them by 

 the penetration of surface waters containing their essential food. 



INORGANIC CHEMICAL AGENCIES. 



The inorganic chemical agencies of decay which attack the rocks in 

 Brazil are carbonic acid (in part) and nitric acid. Nitric and carbonic 

 acids are both brought down by rain ; the latter is also produced by 

 organic decomposition and the former by nitrifying bacteria. 



Carbonic acid (CO.^). — The carbonic acid attacking the rocks of Brazil 

 is derived from two sources : from the air and from the deca}^ of organic 

 matter. The carbonic acid derived from the air is brought down by the 

 rains. It was formerl}'' believed that there was more of this gas in the 

 air (from 4 to G parts by volume in 10,000) than later examinations have 

 shown.^-^ The determinations of Reiset, Van Nuys, Muntz and Aubin 

 and others show conclusively that the amount is small and constant. 

 Examinations in Paris and in the country by Muntz and Aubin showed — 



"That the variations of carbonic acid occur only between very narrow limits and 

 under local influences, so that it may be said in general terms that carbonic acid 

 is uniformly disseminated in the lower strata of the atmosphere." f 



Further determinations by these authors, and described in the paper 

 cited, show that at considerable elevations (2,877 meters on the Pic du 

 Midi in the Pyrenees) the amount of carbonic acid scarcely differs from 

 that in the air near the sea-level. 



Another noteworthy fact is that determinations made in the presence 

 of abundant vegetation showed there was no marked difference. I lay 

 some stress on this because in observing the character and amount of 

 vegetation in Brazil I presumed that the decay of so much organic mat- 

 ter must necessarily return large quantities of carbonic acid and ammo- 

 nia to the air. However much of these substances decaying vegetation 

 may yield, it may be considered as demonstrated that they do not pass 

 into the air, but are carried into the ground by meteoric waters. 



The determinations of the above cited authorities show that the amount 

 of carbonic acid in the air varies only between 2.76 and 3 volumes in 

 10,000, t and this must be regarded as a practical demonstration that the 

 air is not washed clear of carbonic acid by rains, as was formerly sup. 

 posed. The carbonic acid in the atmosphere has been determined at 

 Para. § For reasons given above, however, these results are not regarded 



* Estimations of carbonic acid in the air. T. C. Van Nuys and B. F. Adams. Amer. Chem. 

 Jour., vol. ix, no. 1. 



fSur les proportions d'acide carbonique dans les hautes regions de i'atmospliere. A. Muntz et 

 E. Aubin. Comptes Rendus, 93, 1881, p. 797. 



I Sur I'acide carbonique normal de Pair atmospherique. M. Dumas. Comptes Rendus de 

 TAcademie des Sciences, 94, 1882, pp. 589-594. 



g Thorpe in Jahresbericht der Chemie, 1867, p. 183. 



