252 PRINCIPLES OF CHEMISTRY 



chloric acid, be called * sodium chloride,' then sal-ammoniac, as the 

 product obtained from caustic ammonia or ammonium hydroxide, is- 

 called ' ammonium chloride.' 



The hypothesis that ammoniacal salts correspond with a complex 

 metal ammonium bears the name of the ammonium theory. It was 

 enunciated by the famous Swedish chemist Berzelius after the proposi- 

 tion made by Ampere. The analogy admitted between ammonium and 

 metals is probable, owing to the fact that mercury is able to form an 

 amalgam with ammonium similar to that which it forms with sodium or 

 many other metals. The only difference between ammonium amalgam 

 and sodium amalgam consists in the instability of the ammonium, which 

 easily decomposes into ammonia and hydrogen. 14 Ammonium amalgam 

 may be prepared from sodium amalgam. If the latter be shaken up 

 with a strong solution of sal-ammoniac, the mercury swells up violently 

 And loses its mobility while preserving its metallic appearance. In so 

 doing, the mercury dissolves ammonium that is, the sodium in the 

 mercury is replaced by the ammonium, and replaces it in the sal- 

 ammoniac, forming sodium chloride, NH 4 C1 + HgNa = NaCl + HgNH 4 . 

 Naturally, ammonium amalgam does not entirely prove the existence- 

 of ammonium itself in a separate state ; but it shows the possibility of 

 this substance existing, and, what is more important, its analogy with 

 the metals, because only metals dissolve in mercury, forming compounds, 

 termed * amalgams,' without altering its metallic form. 15 Ammonium 

 amalgam crystallises in cubes, three times heavier than water ; it is- 

 only stable in the cold, and particularly at very low temperatures. It 

 begins to decompose at the ordinary temperature, evolving ammonia 

 and hydrogen in the proportion of two volumes of ammonia and one 



14 Weyl, by working at considerable pressures, obtained the compound NH 3 K, and 

 then ammonium itself by the action of sal-ammoniac on this substance in the form of 

 a blue liquid, but his researches require confirmation. Ammonium amalgam was origi- 

 nally obtained in exactly the same way as sodium amalgam (Davy) ; namely, a piece of 

 sal-ammoniac was taken, and moistened with water (in order to render it a conductor 

 of electricity). A cavity was made in it, into which mercury was poured, and it 

 was laid on a sheet of platinum connected with the positive pole of a galvanic battery, 

 while the negative pole was put into connection with the mercury. On passing a current 

 the mercury increased considerably in volume, and became plastic, while preserving its 

 metallic appearance, just as would be the case were the sal-ammoniac replaced by a, 

 lump of a sodium salt or of many other metals. In the analogous decomposition of 

 common metallic salts, the metal contained in a given salt separates out at the negative 

 pole, immersed in mercury, by which the metal is dissolved. A similar phenomenon in 

 observed in the case of sal-ammoniac ; the elements of ammonium, NH 4 , in this case are 

 also collected in the mercury, and are retained by it for a certain time. 



15 It would seem that hydrogen is also capable of forming an amalgam resembling the 

 amalgam of ammonium. If an amalgam of zinc be shaken up with an aqueous solution of 

 platinum chloride, without access of air, then a spongy mass is formed which easily 

 decomposes, with the evolution of hydrogen. 



