The Carbonates 51 



lated. Calcareous soils contain considerable quantities 

 of these carbonates. 



In pure water these simple carbonates are practically 

 insoluble; but when carbon dioxide is added to the 

 water, they are transformed into bicarbonates* and are 

 readily dissolved, f So the carbonates are leached out 

 of the soils and brought back into the water. So, the 

 solid limestone may be silently removed, or hollowed out 

 in great caverns by little underground streams. So 

 the Mammoth Cave in Kentucky, and others in Cuba, 

 in Missouri, in Indiana and elsewhere on the continent, 

 have been formed. 



The wa,ter gathers up its carbon dioxide in part as it 

 descends through the atmosphere, and in larger part as 

 it percolates thro soil where decomposition is going on 

 and where oxidation products are added to it. 



Carbon dioxide, thus exists in the water in three 

 conditions : ( 1 ) Fixed (and unavailable as plant food) 

 in the simple carbonates; (2) "half -bound" in the 

 bicarbonates; and (3) free. Water plants use first for 

 food, the free carbon dioxid, and then the "half bound" 

 that is in loose combination in the bicarbonates. As 

 this is used up the simple carbonates are released, and 

 the water becomes alkaline. § Birge and Juday have 

 several times found a great growth of the desmid 

 Staurastrum associated with alkalinity due to this 

 cause. In a maximum growth which occurred in 

 alkaline waters at a depth of three meters in Devil's 

 Lake, Wisconsin, on June 15th, 1907, these plants 

 numbered 176,000 per liter of water. 



*CaC03, for example, becoming Ca(HC03)2, the added part of the formula 

 representing a molecule each of CO2 and H2O. 



flf "hard" water whose hardness is due to the presence of these bicarbonates 

 be boiled, the CO2 is driven off and the simple carbonates are re-precipitated (as, 

 for example, on the sides and bottom of a tea kettle). This is "temporary- 

 hardness." "Permanent hardness" is due to the presence of sulphates and 

 chlorides of lime and magnesia, which continue in solution after boiling. 



§Phenolphthalein, being used as indicator of alkalinity. 



