150 CHEMISTRY OF NATURAL WATERS. [IX. 



such that superficial drainage waters in filtering through the 

 soil lose the silica which they held in solution (5). We 

 have further shown that as a result of this tendency to the 

 formation of insoluble silicates, the silicate of soda liberated 

 in the sediments by the decomposition of feldspar generally 

 appears at the surface as carbonate of soda, having been de- 

 composed by earthy carbonates ( 13). 



In two cases, however, considerable quantities of silica are 

 found dissolved in natural waters. The first is met with where 

 the rapid solvent and decomposing action of heated waters 

 is exerted upon alkaliferous silicious minerals ( 14), as seen 

 in springs like the Geysers. The second case is that of those 

 rivers and streams which drain surfaces covered with decaying 

 vegetation and decomposing silicates, from both of which they 

 derive dissolved silica. Such waters contain but small amounts 

 of solid matters, but the proportion of silica is relatively con- 

 siderable, amounting, as we have seen in the water of the 

 Ottawa River, which contains, in 10,000 parts, 0.6116 of 

 solid matters, to 0.2060, or thirty- two per cent; while in 

 the St. Lawrence, which contains, for the same amount of 

 water, 1.6056, the silica equals .3700, or twenty-four per cent, 

 of the solid ingredients. The analysis by H. Ste-Claire De- 

 ville of the river-waters of France show, in like manner, large 

 amounts of silica, which seem to have been hitherto overlooked 

 in the analyses of most chemists. (Ann. de Chim. et Phys. (3), 

 XXIII. 32.) 



It will be seen by a reference to the tables of analyses 

 given in the second part of this paper, that in the waters of 

 the second class the amount of silica is equal to from 0.15 

 to 0.60 parts for 100 of solid matter. In the alkaline waters 

 of the third and fourth classes its proportion is greater, and up 

 to a certain point appears to increase with that of the carbon- 

 ate of soda The amount of silica which these waters con- 

 tain does not in any case exceed one or two ten-thousandths. 



70. Inasmuch as carbonic acid, according to Bischof 

 (Chem. Geol., I. 2), decomposes not only the silicates of soda, 

 but those of liine and magnesia, when they are in solu- 





