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was by day from 33° to 34° of the centigrade 

 thermometer; they exhaled a strong* smell of 

 sulphuretted hydrogen, to which no doubt some 

 rotten plant of arum and heliconia, that swam 

 on the surface of the pools, contributed. The 

 waters of Lagartero were of a golden yellow by 

 transmitted, and coffee brown by reflected light. 

 They are no doubt coloured by a carburet of 

 hydrogen. An analogous phenomenon is ob- 

 served in the dunghill waters prepared by our 

 gardeners, and in the waters that issue from 

 bogs. May we not also admit, that it is a 

 mixture of carbon and hydrogen, an extractive 

 vegetable matter, that colours the black rivers, 

 the Atabapo, the Zama, the Mataveni, and the 

 Guainia ? The frequency of the equatorial rivers 

 contributes no doubt to this coloration by nitra- 

 tions through a thick wad of grasses. I sug- 

 gest these ideas only in the form of a doubt. 

 The colouring principle seems to be in very 

 little abundance ; for I observed, t hat the waters 

 of the Guainia or Rio Negro, when subjected to 

 ebullition, do not become brown like other 

 fluids charged with carburets of hydrogen. 



It is also very remarkable, that this pheno- 

 menon of black waters, which might be supposed 

 to belong only to the low regions of the torrid 

 zone, is found also, though rarely, on the table- 

 lands of the Andes. The town of Cuenca in the 

 Kingdom of Quito, is surrounded by three small 



