carbonic acid (for instance, NaHCO 3 ), and at 38° the separation of carbonic anhydride takes place with great rapidity. On losing carbonic anhydride and water, the acid salt is converted into the normal salt, 2(NH 4 )HCO 3 = H 2 O + CO 2 + (NH 4 )2CO 3 ; the latter, however, decomposes in solution, and can therefore only be obtained in crystals, (NH 4 ) 2 CO 3 ,H 2 O, at low temperatures, and from solutions containing an excess of ammonia as the product of dissociation of this salt: (NH 4 ) 2 CO 3 = NH 3 + (NH 4 )HCO 3 . But the normal salt, according to the general type, is capable of decomposing with 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 chemical transformations of the carbonates of ammonium. 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 Cl + CaCO 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 + 3CO 2 + 2H 2 O. This salt, in parting under various conditions 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; 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 corresponding 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 OHCONH2, or carbamic acid, is formed from OHCO3NH4. This acid is not known in a free state, but its corresponding ammoniacal salt or ammonium carbamate is known. The latter is easily and immediately formed by mixing 2 volumes of dry ammonia with 1 volume of dry carbonic anhydride, 2NH3 + CO2 = NH4O(CONH2); 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 proved44 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 of carbonic anhydride. It is easily understood that such a combination will take place with any ammonium carbonate under the action of salts which take up the water—for instance, sodium or potassium carbonate45—as in an anhydrous state ammonia and carbonic anhydride only form one compound, CO22NH3.46 As the normal ammonium carbonate contains two ammonias, and as the amides are formed with the separation of water at the expense of the hydrogen of the ammonias, so this salt has its symmetrical amide, CO(NH2)2. This must be termed carbamide. It is identical with urea, CN2H4O, which, contained in the urine (about 2 per cent. in human urine), is for the higher animals (especially the carnivorous) the ordinary product of excretion47 and oxidation of the nitrogenous substances found in the organism. If ammonium carbamate be heated to 140° (in a sealed tube, Bazaroff), or if carbonyl chloride, COCl2, be treated with ammonia (Natanson), urea will be obtained, which shows its direct connection with carbonic acid—that is, the presence of carbonic acid and ammonia in it. From this it will be understood how urea during the putrefaction of urine is converted into ammonium carbonate, CN2H4O + H2O = CO2 + 2NH3.
Thus urea, both by its origin and decomposition, is an amide of carbonic acid. Representing as it does ammonia (two molecules) in which hydrogen (two atoms) is replaced by the bivalent radicle of carbonic acid, urea retains the property of ammonia of entering into combination, with acids (thus nitric acid forms CN 2 H