COMI'olNDS OF NITROGEN AVJTH HYDROGEN AND OXYGEN 253 



volume of hydrogen, NH 4 = NH 3 + H. By the action of water, 

 ammonium amalgam gives hydrogen and ammonia water, just as 

 .sodium amalgam gives hydrogen and sodium hydroxide ; and therefore, 

 in accordance with the ammonium theory, ammonia water must be 

 looked on as containing ammonium hydroxide, NH,OH, 1G just as an 

 aqueous solution of sodium hydroxide contains NaOH. The ammonium 

 hydroxide, like ammonium itself, is an unstable substance, which easily 

 dissociates, and which can only exist in a free state at low tempera- 

 tures. 17 Ordinary solutions of ammonia must be looked on as the 

 products of the dissociation of this hydroxide, inasmuch as NH 4 OH 

 = NH 3 + H 2 O. The liability to a greater or less decomposition is 

 proper, in the same degree, to substances containing NH 3 as to 

 substances containing water. 



All ammoniacal salts decompose at a red heat into ammonia and an 

 acid, which, on cooling in contact with each other, re-combine together. 

 If the acid be non-volatile, the ammoniacal salt, when heated, evolves 

 the ammonia, leaving the non-volatile acid behind ; if the acid be vola- 

 tile, then, on heating, both the acid and ammonia volatilise together, 

 and on cooling re-combine into the salt which originally served for the 

 formation of their vapours. 18 



Ammonia is not only capable of combining with acids, but also 

 Avith many salts, as was seen from its forming definite compounds, 

 AgCl,3NH 3 and 2AgCl,3KH 3 , with silver chloride. So, also, am- 

 monia is absorbed by the chlorine, iodine, and bromine compounds 

 of many metals, and in so doing evolves heat. Certain of these com- 



16 We saw above that the solubility of ammonia in water at low temperatures 

 attains to the molecular ratio NH 3 + H 2 O, in which these substances are contained in 

 caustic ammonia, and perhaps it may be possible at exceedingly low temperatures to 

 obtain ammonium hydroxide, NH 4 HO, in a solid form. By regarding solutions as disso- 

 ciated definite compounds, we should see a confirmation of this view in the property 

 shown by ammonia of being extremely soluble in water, and in so doing of approaching 

 to the limit NH 4 HO. 



17 In confirmation of the truth of this conclusion we may cite the remarkable fact 

 that there exist, in a free state and as comparatively stable compounds, a series of alka- 

 line hydroxides, NR 4 HO, which are perfectly analogous to ammonium hydroxide, and 

 present a striking resemblance to it and to sodium hydroxide, with the only difference 

 that the hydrogen in NH 4 HO is replaced by complex groups, E = CH 3 , C 2 H 5 , &c., for 

 instance N(CH 3 ) 4 HO. 



18 The fact that ammoniacal salts are decomposed when ignited, and not simply 

 sublimed, may be proved by a direct experiment with sal-ammoniac, NH 4 C1, which in a 

 state of vapour is decomposed into ammonia, NH 3 , and hydrochloric acid, HC1, as will 

 be explained in the following chapter. The readiness with which ammonium salts decom- 

 pose is seen from the fact that a solution of ammonium oxalate is decomposed with the 

 evolution of ammonia even at 1. Dilute solutions of ammonium salts, when boiled, 

 give aqueous vapour, having an alkaline reaction, owing to the presence of free ammonia 

 given off from the salt. 



