COMPOUNDS OF CABBON WITH OXYGEN AND NITROGEN 391 



nate, CaC0 3 . Only the normal (not the acid) salts of such powerful bases 

 as potassium and sodium are capable of standing a red heat without 

 decomposition. The acid salts for instance, NaHCO 3 decompose 

 even on heating their solutions (2NaHC0 3 = Na 2 C0 3 + H 2 + CO 2 ), 

 evolving carbonic anhydride. The amount of heat given out by the 

 combination of carbonic acid with bases also shows its feeble acid 

 properties, being considerably less than with energetic acids. Thus 

 if a weak solution of forty grams of sodium hydroxide be satu- 

 rated (up to the formation of a normal salt) with sulphuric or nitric 

 acid or another powerful acid, from thirteen to fifteen thousand 

 calories are given out, but with carbonic acid only about ten thousand 

 calories. 16 The majority of carbonates are insoluble in water, and 

 therefore such solutions as sodium, potassium, or ammonium carbonates 

 form in solutions of most other salts, MX or M"X 2 , insoluble pre- 

 cipitates of carbonates, M 2 CO 3 or M"C0 3 . Thus a solution of barium 

 chloride gives with sodium carbonate a precipitate of barium carbonate, 

 BaCO 3 . For this reason rocks, especially those of aqueous origin, very 

 often contain carbonates ; for example, calcium, ferrous, or magnesium 

 carbonates, &c. 



Carbonic anhydride which, like water, is formed with the develop- 

 ment of a large amount of heat is very stable. Only very few sub- 

 stances are capable of depriving it of its oxygen. However, certain 

 metals, such as magnesium, potassium and the like, on being heated, burn 

 in it, depositing carbon and forming oxides. If a mixture of carbonic 

 anhydride and hydrogen be passed through a heated tube, the formation 

 of water and carbonic oxide will be observed ; C0 2 + H 2 = CO -H H 2 O. 



16 Although carbonic acid is reckoned among the feeble acids, yet there are evi- 

 dently many others- still feebler for instance, prussic acid, hypochlorous acid, many 

 organic acids, &c. Bases like alumina, or such feeble acids as silica, when in combination 

 with alkalis, are decomposed in aqueous solutions by carbonic acid, but on fusion that 

 is, without the presence of water they displace it, which clearly shows in phenomena oJ 

 this kind how much depends upon the conditions of reaction and the properties of the 

 substances formed. These relations, which at first sight appear complex, may be best 

 understood if we represent t,hat two salts, MX and NY, in general always give more or 

 less of two other salts, MY and NX, and then examine the properties of the derived sub- 

 stances. Thus, in solution, sodium silicate, Na.>SiO 3 , with carbonic anhydride will to some 

 extent form sodium carbonate and silica, SiO 2 ; but the latter, being colloid, separates, and 

 the remaining mass of sodium silicate is again decomposed by carbonic anhydride, so that 

 finally silica separates and sodium carbonate is formed. In a fused state the case is 

 different ; sodium carbonate will react with silica to form carbonic anhydride and sodium 

 silicate, but the carbonic anhydride will be separated as a gas, and therefore in the 

 residue the same reaction will again take place, and ultimately the carbonic anhydride is 

 entirely eliminated and sodium silicate remains. If, on the other hand, nothing is removed 

 from the sphere of the reaction, distribution takes place. Therefore, although car- 

 bonic anhydride is a feeble acid, still not for this reason, but only in virtue of its gaseous 

 form, do all soluble acids displace it in aline solutions (see Chapter X.) 



*5 



