244 PRINCIPLES OF CHEMISTRY 



lation, they decompose. A portion of the substances proceeding 

 from the decomposition remains in the retort and forms a carbonaceous 

 residue, whilst the other portion, in virtue of its volatility, escapes 

 through the tube leading from the retort. The vapours given off, on 

 cooling, form a liquid which separates into two layers ; the one, which 

 is oily, is composed of the so-called animal oils (oleum animate), the 

 other, an aqueous layer, contains a solution of ammonia salts. If this 

 solution be mixed with lime and heated, the lime takes up the elements 

 of carbonic acid from the ammonia salts, and ammonia is evolved as a 

 gas. 3 In ancient times ammonia compounds were imported into Europe 

 from Egypt, where they were prepared from the soot obtained in the 

 employment of camels' dung as fuel in the locality of the temple of 

 Jupiter Ammon (in Lybia), and therefore the salt obtained was called 

 'sal-ammoniacale,' from which the name of ammonia is derived. Now 

 ammonia is exclusively obtained, on a large scale, either from the products 

 of the dry distillation of animal or vegetable refuse, from urine, or from 

 the ammoniacal liquors collected in the destructive distillation of coal 



into nitrogen and hydrogen. This is a phenomenon of dissociation, as was explained in the 

 preceding chapter, p. 227. Therefore, a series of sparks do not totally decompose the 

 ammonia, but leave a certain portion undecomposed. One volume of nitrogen and three 

 volumes of hydrogen are obtained from two volumes of ammonia. The presence of free 

 ammonia that is, ammonia not combined with acids in a gas or aqueous solution may 

 be recognised by its characteristic smell. But many ammonia salts do not possess this 

 smell. However, on the addition of an alkali (for instance, caustic lime, potash, or soda), 

 they evolve ammonia gas, especially when heated. The presence of ammonia may be 

 made visible by introducing a substance moistened with strong hydrochloric acid into 

 its neighbourhood. A white cloud, or visible white vapour, then makes its appearance. 

 This depends on the fact that both ammonia and hydrochloric acid are volatile, and 

 on coming into contact with each other form solid sal-ammoniac, NH 4 C1, which forms a 

 cloud. This test is usually made by dipping a glass rod into hydrochloric acid, and* 

 holding it over the vessel from which the ammonia is evolved. With small amounts of 

 ammonia this test is, however, untrustworthy, as the white vapour is scarcely observable. 

 In this case it is best to take paper moistened with mercurous nitrate, HgNO 3 . This 

 paper turns black in the presence of ammonia, owing to the formation of a black com- 

 pound of ammonia with mercurous oxide. The smallest traces of ammonia, for instance, 

 in river water, may be discovered by means of the so-called Nessler's reagent, containing 

 a solution of mercuric chloride and potassium iodide, which forms a brown coloration 

 or precipitate with the smallest quantities of ammonia. Here it will be useful to give the 

 thermo-chemical data (in thousands of units of heat, according to Thomsen), or the 

 quantities of heat evolved in the formation of ammonia and its compounds in quantities 

 expressed by their formulae. Thus, for instance (N + H 3 ) 26'7 indicates that 14 grams 

 of nitrogen in combining with 3 grams of hydrogen develop sufficient heat to raise the 

 temperature of 26'7 kilograms of water 1. (NH 5 + nH 2 O) 8'4 (heat of solution); 

 (NH 3 ,nH 2 + HCl,nH 2 0)12-8; (N + H 4 + Cl) 90'6 ; (NH 3 + HC1) 41'9. 



5 The same ammonia water is obtained, although in smaller quantities, in the 

 dry distillation of plants and of coal, which consists of the remains of fossil plants. 

 In all these cases the ammonia proceeds from the destruction of the complex nitrogenous 

 substances occurring in plants and animals. The ammonia salts employed in practice 

 are prepared by this method. 



