408 PRINCIPLES OF CHEMISTRY 



capable of decomposing with separation of water, and forming ammonitim 

 carbamate, NH 4 0(CONH 2 ) = (NH 4 ) 2 C0 3 - H 2 O ; this still further 

 complicates the chemical transformations of the carbonates of am- 

 monium. It is in fact evident that, by changing the ratios of 

 water, ammonia, and carbonic acid, various intermediate salts will be 

 formed containing mixtures or combinations of those mentioned above. 

 Thus the ordinary commercial carbonate of ammonia is obtained by 

 heating a mixture of chalk and sulphate of ammonia (Chapter VI.), or 

 sal-ammoniac, 2NH 4 C1 + CaC0 3 = CaCl 2 + (NH 4 ) 2 CO 3 . The normal 

 salt, however, through loss of part of the ammonia, partly forms the 

 acid salt, and, partly through loss of water, forms carbamate, and most 

 frequently presents the composition NH 4 O(CONH 2 ) + 2OH(CO 2 NH 4 ) 

 = 4NH 3 4- 3C0 2 + 2H 2 O. This salt, in parting under various con- 

 ditions 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 ; 35 but on being dissolved in water this salt gives a mixture of acid 

 and normal salts. 



Each of the two ammoniacal salts of carbonic acid has its corre- 

 sponding amide. That of the acid salt should be acid, if the water given 

 off takes up the hydrogen of the ammonia, as it should according to the 

 common type of formation of the amides, so that OHCONH 2 , or 

 carbamic acid, is formed from OHCO 3 NH 4 . This acid is not known in 

 a free state, but its corresponding ammoniacal salt or ammonium car- 

 bamate is known. The latter is easily and immediately formed by 

 mixing 2 volumes of dry ammonia with 1 volume of dry carbonic anhy- 

 dride, 2NH 3 -f CO 2 = NH 4 0(CONH 2 ) ; it is a solid substance, smells 

 strongly of ammonia, attracts moisture from the air, and decomposes 

 completely at 60. The fact of this decomposition may be proved 36 by 

 the density of its vapour, which = 13 (H = 1) ; this exactly corresponds 

 with the density of a mixture of 2 volumes of ammonia and 1 volume 



55 In the normal salt, 2NH 3 + CO 2 + H 2 O, in the acid salt, NH 3 + CO 2 + H 2 O, but in the 

 commercial salt only 2H 2 O to 3CO 2 . 



36 Naumann determined the following dissociation tensions of the vapour of ammonium 

 Cftrbamate (in millimetres of mercury) : 



-10 +10 20 80 40 50 60 



5 12 80 62 124 248 470 770 



Horstmann and Isambert studied the tensions corresponding to excess of NH 3 or CO 2 , 

 and found, as might have been expected, that with such excess the mass of the salt 

 formed (in a solid state) increases and the decomposition (transition into vapour) 

 decreases. 



