458 PRINCIPLES OF CHEMISTRY 



of equal volumes 6f each is immediately formed. The same com* 

 pound is obtained on mixing solutions of the two gases. It is also 

 produced by the action of hydrochloric acid on ammonium carbonate. 

 Sal-ammoniac is usually prepared, in practice, by the last method. 4 * 

 The specific gravity of sal-ammoniac is 1'55. We have already seen 

 (Chapter VI.) that sal-ammoniac, like all other ammonium salts, easily 

 decomposes ; for instance, by volatilisation -with alkalis, and even 

 partially when its solution is boiled. The other properties and 

 reactions of sal-ammoniac, especially in solution, fully recall 

 those already mentioned in speaking of sodium chloride. Thus, 

 for instance, with silver nitrate it gives a precipitate of silver chloride } 

 with sulphuric acid it gives hydrochloric acid and ammonium sulphate, 

 and it forms double salts with certain metallic chlorides and other 

 salts. 45 



ammonium should be understood in this way NH 3 gives NH 4 C1 because NX 3 is 

 capable of giving NX 5 . But as saturated compounds for instance, SO 3 H 2 O, NaCl, 

 &c. are also capable of combination even between themselves, it is impossible 

 to deny the capacity of HC1 also for combination. SOj combines with HjO, and also with 

 HC1 and the unsaturated hydrocarbons. It is impossible to recognise the distinction 

 formerly sought to "be established between atomic and molecular compounds, and 

 regarding, for instance, PC1 5 as an atomic compound and PCl 5 as a molecular one, only 

 because it easily splits up into molecules PCls and C1 2 . 



44 Sal-ammoniac is prepared from ammonium carbonate, obtained in the dry distilla- 

 tion of nitrogenous substances (Chapter VI.), by saturating the resultant solution with 

 hydrochloric acid. A solution of sal-ammoniac is thus produced, which is evaporated, 

 and in the residue a mass is obtained containing a mixture of various other, especially 

 tarry, products of dry distillation. The sal-ammoniac is generally purified by sublima- 

 tion. For this purpose iron vessels covered with hemispherical metallic covers are 

 employed, or else simply clay crucibles covered by other crucibles. The upper portion, 

 or head, of the apparatus of this kind will have a lower temperature than the lower por- 

 tion, which is under the direct action of the flame. The sal-ammoniac volatilises when 

 heated, and settles on the cooler portion of the apparatus. It is thus freed from many 

 impurities, and is obtained as a crystalline crust, generally several centimetres thick, in 

 which form it is commonly sold. The solubility of sal-ammoniac rises rapidly with the 

 temperature : at 0, 100 parts of water dissolve about 28 parts of NH 4 C1, at 50 about 

 60 parts, and at the ordinary temperature about 85 parts. This is sometimes takefc 

 advantage of for separating NH^Cl from solutions of other salts. 



45 The solubility of sal-ammoniac in 100 parts. of water (according to Alluard) is 



10 20 80 40 60 80 100 110 



28-40 32-48 37'28 41'72 46 55 61 73 77 



A saturated solution boils at 115-8. The specific gravity at 15/4 of solutions of sal- 

 ammoniac (water 4 = 10,000) =9,991 < 6 + 8r26p 0'085jp 2 , where p is the amount by weight 

 of ammonium chloride in 100 parts of solution. With the majority of salts the differen- 

 tial da/dp increases, but here it decreases with the increase of p. For (unlike the 

 sodium and potassium salts) a solution of the alkali plus a solution of acid occupy a 

 greater volume than that of the resultant ammonium salt. In the solution of solid 

 ammonium chloride a contraction, and not expansion, generally takes place. It may 

 further be remarked that solutions of sal-ammoniac have an acid reaction even when 

 prepared from the salt remaining after prolonged washing of the sublimed salt with 

 wafer CA^ StcherbfikofiL 



