396 PRINCIPLES OF CHEMISTRY 



instance, NaHCO 3 ), and at 38 the separation of carbonic anhydride 

 takes place with great rapidity. For this reason the formation of the 

 acid salt in the solution, and the evaporation, must be conducted at a 

 low temperature, and with an excess of carbonic acid, as the product of 

 dissociation of this salt. On losing carbonic anhydride and water,, 

 the acid salt is converted into the normal salt, 2(NH 4 )HCO 3 = 

 H 2 O4-CO 2 -f-(N"H 4 ) 2 CO 3 ; the latter, however, decomposes in solution,, 

 and therefore can only be obtained in crystals, (NH 4 ) 2 CO 3 ,H 2 O, at 

 low temperatures, and from solutions containing an excess of ammo- 

 nia as the product of dissociation of this salt : (NH 4 ) 2 CO 3 = 

 NH 3 + (NH 4 )HCO 3 . But the normal salt, 34 according to the general 

 type, is capable of decomposing ivith separation of water, and forming 

 ammonium carbamate, NH 4 O(CONH 2 )=(NH 4 ) 2 CO 3 H 2 O ; this still 

 further complicates the history of the carbonates of ammonium. It is. 

 evident that, in reality, on changing the quantity of water, ammonia, 

 and carbonic acid, various intermediate salts will be formed con- 

 taining mixtures or combinations of those mentioned above, or, in 

 other words, various states of equilibrium of reactions reverse to one 

 another will be produced. Thus the ordinary commercial carbonate of 

 ammonia, obtained by distillation of a mixture of chalk and sulphate 

 of ammonia (Chapter VI.), or sal-ammoniac, 2NH 4 Cl + CaCO 3 = 

 CaCl 2 + (NH 4 ) 2 CO 3 , does not remain as a normal salt, but, through loss- 

 of part of the ammonia, partly contains the acid salt, and, through loss- 

 of water, contains some carbamate, and most frequently presents the coin- 

 position NH 4 0(CONH 2 ) + 2OH(CO 2 NH 4 ) = 4NH 3 + 3C0 2 + 2H 2 O. 

 Indeed, this salt, from having under various conditions parted with 

 ammonia, carbonic anhydride, and water, does not present a constant 

 composition, and ought rather to be regarded as a mixture of acid salt 

 and amide salt. The latter must be recognised as entering into the 

 composition of the ordinary carbonate of ammonia, because it contains, 

 less water than is required for the normal or acid salt ; 3S but on being 

 dissolved in water this salt gives a mixture of acid arid normal salts. 

 Ammonium carbamate itself, whose composition (and formation) is. 

 equal to 2NH 3 + CO 2 , when dissolved in water, does not entirely 

 precipitate a calcium salt, for instance CaCl_ ; , which produces a normal 

 salt, (NH 4 ) 2 CO 3 + CaCl 2 = 2NH 4 Cl + CaCO 2 , probably because a calcium 

 carbamate is produced which is soluble in water. 



31 The acid salt (NH 4 )HCO 3 on losing water ought to form the carboxylic acid, 

 (NH 2 )HCO 2 , or, better, OH(CNH 2 O), as we wrote it before ; but it is not formed, which 

 is accounted for by the instability of the acid salt itself. Carbonic anhydride is given 

 off and ammonia is produced, which gives a carboxylic ammonium salt. 



35 In the normal salt, 2NH 3 + CO 2 + H a O, in the acid salt, NH 3 + CO 2 + H.,O, but in the 

 commercial salt to 3CO 2 only 2H 2 O 



