246 PRINCIPLES OF CHEMISTRY 



perties. Ammonia may be very easily prepared from this sal-ammoniac, 

 NH 4 C1, as from any other ammoniacal salt, by heating it with lime. 

 Calcium hydroxide, CaH 2 O 2 , as an alkali takes up the acid and sets 

 free the ammonia, forming calcium chloride, according to the equation 

 2NH 4 Cl + CaH 2 O 2 = 2H 2 O + CaCl 2 + 2NH 3 . I n this reaction the 

 ammonia, as a gas, is evolved. 6 



It must be observed that all the complex nitrogenous substances of 

 plants, animals, and soils are decomposed when heated with an excess 

 of sulphuric acid, the whole of their nitrogen being converted into 

 ammonium sulphate, from which it may be liberated by treatment with 

 an excess of alkali. This reaction is so complete that it forms the 

 basis of Kjeldahl's method for estimating the amount of nitrogen in its 

 compounds. 



Ammonia is a colourless gas, resembling those with which we are 

 already acquainted in its outward appearance but clearly distinguish- 

 able from any other gas by its very characteristic and strong smell. It 

 irritates the eyes, and it is positively impossible to inhale it. Animals 

 die in it. Its density, referred to hydrogen, is 8-5 ; hence it is lighter 

 than air. It belongs to the class of gases which are easily liquefied. 7 



6 On a small scale ammonia may be prepared in a glass flask by mixing equal parts 

 by weight of slacked lime and finely-powdered sal-ammoniac, the neck of the flask 

 being connected with an arrangement for drying the gas obtained. In this instance 

 neither calcium chloride nor sulphuric acid can be used for drying the gas, because 

 they absorb ammonia, and therefore solid caustic potash, which is capable of retaining 

 the water, is employed. The gas conducting tube leading from the desiccating apparatus 

 is introduced into a mercury bath, if dry gaseous ammonia be required, because water 

 cannot be employed in collecting ammonia gas. Ammonia was first obtained in this dry 

 state by Priestley, and its composition was investigated by Berthollet at the end of the 

 last century. Oxide of lead mixed with sal-ammoniac (Isambert) evolves ammonia 

 with still greater ease than lime. The cause and process of the decomposition is almost 

 the same, 2PbO -t- 2NH 4 C1 = Pb 2 OCl 2 + H 2 O + 2NH 3 . Lead oxychloride is (probably 

 formed. 



7 This is evident from the fact that its absolute boiling point lies about + 180 (Chap 

 II. Note 29). Consequently, it may be liquefied by pressure alone at the ordinary, and even 

 much higher, temperatures. The latent heat of evaporation of 17 parts by weight 

 of ammonia equals 4400 units of heat, and therefore liquid ammonia may be employed 

 for the production of cold. Strong aqueous solutions of ammonia, which in parting with 

 their ammonia act in a similar manner, are not unfrequently employed for this purpose^ 

 Suppose a saturated solution of ammonia to be held in a closed vessel furnished with 

 a receiver. If the ammoniacal solution be heated, the ammonia, with a small quantity 

 of water, will pass off from the solution, and in accumulating in the apparatus will 

 produce a considerable pressure, and will therefore liquefy in the cooler portions of the 

 receiver. Hence liquid ammonia will be obtained in the receiver. The heating of the 

 vessel containing the aqueous solution of ammonia is then stopped. After having been 

 heated it contains only water, or a solution poor in ammonia. When once it begins to cool, 

 the ammonia vapours commence dissolving in it, the space becomes rarefied, and a rapid 

 vaporisation of the liquefied ammonia left in the receiver takes place. In evaporating in 

 the receiver it will cause the temperature in it to fall considerably, and will itself pass into 



