INFLUENCE OF RAINFALL. 309 



acids after standing for some time. Nitrification takes place principally 

 if not exclusively in the surface soil.^ 



In order to estimate the nitric acid in the ammonia falling in rain in 

 Brazil we are compelled to get our ideas of the amount in the rain-water 

 from the determinations of Muntz and Marcano at Caracas, Venezuela. 

 The total ammonia falling in the rain at that place on being oxidized to 

 nitric acid amounts to 6.975 milligrams of nitric acid to the liter of rain- 



water.f 



Thus the table on page 308 gives the total depth of nitric acid fall- 

 ing in the rains in Brazil, both in the form of ammonia and as nitric 

 acid. 



BAIN FALL. 



It is evident that the rainfall must be an important factor in all this 

 rock decomposition ; both the amount and the time distribution are im- 

 portant elements. At one of the stations from which we have a record — 

 that of the Alto da Serra do Cubatao, where the Santos a Jundiahy rail- 

 way crosses the Serra between Santos and Sao Paulo — an average for 

 15 years gives a rainfall of 3,576.7 millimeters (140.81 inches, or more than 

 11 feet), and from this extreme it declines to about 50 inches. Moreover, 

 the -rainfall is very unevenly distributed throughout the year, most of the 

 precipitation occurring in three or four months. This same precipita- 

 tion, large as it is, if more evenly distributed throughout the year would 

 do only a fraction of the eroding that it does when thus poured in torrents 

 upon the earth.J 



The year is roughly divided by the people into the two seasons which 

 are known as the sunny weather (tempo de sol) and rainy weather (tempo 

 de chuva). The tempo de sol is the time of cool weather in that country — 

 usually the months of May, June, July, August and September — the sol 

 referring not to the heat, but to the continuity of the sunshine. The 

 rainy season is the hot part of the 3^ear, and this is a point to be borne 

 in mind, for the rains alternating with hot sunshine, the waters fall upon 

 hot rocks or soils and their chemical activity is greatly increased by this 

 increase of temperature. 



The concentration of the rainfall in a few months of the year is a con- 

 stant feature of the Brazilian climate, although it often varies consider- 

 ably from one year to another — that is to say, November may be a very 

 rainy month one year and comparatively little rain may fall the follow- 



* Warington : Jour. Chem. Soc, vol. li, p. 118. 

 fComptes Rendus, 1891, exiii, p. 780. 



t Lake Bonneville. G.K.Gilbert. Monogi-aph I, U. S. Geol. Survey, pp. 41, 42, This principle is 

 employed in the process of gold washing known as "booming." 



