CARBONIC ACID. 



307 



present in the atmosphere in a sensible although by no means 

 considerable proportion, (page 280.) 



Carbonic acid combines with bases, and forms the class of 

 carbonates. The hydrate of this acid seems incapable of existing 

 in an uncombined state, but it exists in the alkaline bicarbo- 

 nates, which are double carbonates of water and the alkali. 

 If this hydrate were formed, it would probably be found ana- 

 logous to the crystallized carbonate of magnesia, of which the 

 formula is MgO, CO 2 + HO-f2HO, and also the same with 

 another 2HO; the salt of magnesia of most acids resembling the 

 salt of water. Carbonate of lime, in the hydrated condition, has 

 a similar formula. But the carbonates exhibit little affinity for 

 water, and are generally anhydrous. Those of the alkalies retain 

 a strong alkaline reaction, owing to the weakness of this acid, 

 and the carbonates generally are decomposed with effervescence 

 by all other acids, except hydrocyanic. 



Uses. Carbonic acid is not used in the arts, except in the 

 preparation of aerated waters. The strong vessels in which the 

 impregnation is effected, should be of copper well tinned, and 

 not of iron, as with the concurrence of water carbonic acid acts 

 strongly upon that metal. It is sometimes desirable to remove 

 carbonic acid from air or other gaseous mixtures, and this is 

 generally done by means of caustic alkali or lime-water. When 

 very dry, or so humid as to be actually wet, the hydrate of lime 

 absorbs this gas with much less avidity than when of a certain 

 degree of dryness, in which it is not so dry as to be dusty, but 

 at the same time not sensibly damp. The dry hydrate may be 

 brought at once to this condition, by mixing it intimately with 

 an equal weight of glauber's salt, in fine powder ; and this mix- 

 ture in a stratum of not more than an inch in thickness inter- 

 cepts carbonic acid most completely, and may rise in tempera- 

 ture to above 200 froni the rapid absorption of the gas. It is 

 quite possible to respire through a cushion of that thickness, 

 filled with this mixture, and such an article might be found 

 useful by parties entering an atmosphere overcharged with car- 

 bonic acid, like that of a coal mine after the occurrence of an 

 explosion of fire damp. 



Carbonic acid is the highest degree of oxidation of which car- 

 bon is susceptible; but another oxide of carbon exists contain- 

 ing less oxygen. 



x 2 



