349 



not justify the ancient hypothesis of the formal 

 tion of water in the air by the combination of 

 oxygen and hydrogen. In vain has hydrogen 

 been sought as far as three thousand six hun- 

 dred toises of height. The quantity of water 

 contained in saturated air augments much more 

 rapidly from twenty to twenty- five degrees than 

 from ten to fifteen degrees. A single degree of 

 cooling produces consequently a greater quan- 

 tity of visible vapours in the torrid than in the 

 temperate zone. Air unceasingly renewed by 

 the effect of currents may furnish by simple 

 precipitation all the water, which so much 

 strikes the imagination of philosophers in the 

 equatorial rains. 



The colour of the water of the Rio Negro is 

 (by reflection) darker than that of the Atabapo 

 or the Tuamini. I even saw with surprise, how 

 little the mixture of the white waters of the 

 Cassiquiare alters the tint below the fort of San 

 Carlos. The author of the modern Chorography 

 of Brazil justly observes*, that the river is of an 

 amber colour, wherever it is shallow, and of a 

 dark brown like coffee grounds, wherever the 

 depth of the waters is considerable. The name 

 of Curana, which is given by the natives to the 



* Vol. ii, p. 336. We may be surprised to find M. de la 

 Condamine attributing the denomination of Rio Negro " to 

 the great transparency of that sea of fresh water, which the 

 Amazon receives near the fort of Barra." 



