CARBON AND CARBONIC ANHYDRIDE. 89 



of a class of bodies which unite with water to form acids. 

 You have learned in 70 that nitric anhydride combines 

 with water, forming nitric acid ; thus N 2 O 5 -f-H 2 O=2HNO 3 . 

 But carbonic acid does not seem to have any definite ex- 

 istence ; the solution of CO 2 in H 2 O may be regarded as 

 H 2 CO 3 , but the acid, if it exist at all, is decomposed at ordi- 

 nary temperatures, and quickly by boiling. Although car- 

 bonic acid has a doubtful existence, the carbonates derived 

 from it are a most important and numerous class of bodies. 

 From these carbonates the stronger acids, hydrochloric and 

 nitric, for example (or even acetic), do not drive out carbon- 

 ic acid, but carbonic anhydride, and the reaction is accom- 

 panied by the formation of water. 



111. An Apparent Inconsistency. Notwithstanding the 

 comparative weakness of carbonic acid, it adheres to its 

 union with some substances in spite of the most intense 

 heat. No degree of heat can drive off the carbonic acid 

 from potassium or sodium carbonates. Why this is we 

 know not, neither do we know why lime will take carbonic 

 acid away from potash. It would seem from this that car- 

 bonic acid has a stronger attraction for calcium than for 

 potassium, and yet, from the effect of heat upon carbonate 

 of lime, we should make exactly the opposite inference. 

 This is one of the apparent contradictions in affinity which 

 we can not explain, though the object which the Creator 

 had 'in making such differences is generally quite obvious. 



112. Carbonic Oxide. This is a gas which has but half 

 as much oxygen in it as carbonic anhydride, its formula be- 

 ing CO. It is a transparent colorless gas, and burns with 

 a beautiful blue flame, which you have often seen playing 

 over the surface of an anthracite fire as it kindles. The ex- 

 planation of its formation in this case is this : The closely 

 packed coal does not get sufficient oxygen from the air to 

 make carbonic anhydride, and so carbonic oxide is formed. 



