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 4 OH, just as an aqueous solution of sodium hydroxide, contains NaOH. The ammonium hydroxide, like ammonium itself, is an unstable substance, which easily dissociates, and can only exist in a free state at low temperatures. 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.
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 volatile, 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 with many salts, as was seen from its forming definite compounds, AgCl,3NH3 and 2AgCl,3NH3, with silver chloride. Just as ammonia is absorbed by various oxygen salts of the metals, so also is it absorbed by the chlorine, iodine, and bromine compounds of many metals, and in so doing evolves heat. Certain of these compounds part with their ammonia even when left exposed to the air, but others only do so at a red heat; many give up their ammonia when dissolved, whilst others dissolve without decomposition, and when evaporated separate from their solutions unchanged. All these facts only indicate that ammoniacal, like aqueous, compounds dissociate with greater or lesser facility.19 Certain metallic oxides also absorb ammonia and are dissolved in ammonia water. Such are, for instance, the oxides of zinc, nickel, copper, and many others; the majority of such compounds are unstable. The property of ammonia of combining with certain oxides explains its action on certain metals.20 By reason of such action, copper vessels are not suitable for holding liquids containing ammonia. Iron is not acted on by such liquids.
The similarity between the relation of ammonia and water to salts and other substances is more especially marked in those cases in which the salt is capable of combining with both ammonia and water. Take, for example, copper sulphate, CuSO4. As we saw in Chapter I., it gives with water blue crystals, CuSO4,5H2O; but it also absorbs ammonia in the same molecular proportion, forming a blue substance, CuSO4,5NH3, and therefore the ammonia combining with salts may be termed ammonia of crystallisation.
Such are the reactions of combination proper to ammonia. Let us now turn our attention to the reactions of substitution proper to this substance. If ammonia be passed through a heated tube containing metallic sodium, hydrogen is evolved, and a compound is obtained containing ammonia in which one atom of hydrogen is replaced by an atom of sodium, NH 2 Na (according to the equation NH 3 + Na = NH 2 Na + H). This body is termed sodium amide. We shall afterwards see that iodine and chlorine are also capable of directly displacing hydrogen from ammonia, and of replacing it. In fact, the hydrogen of ammonia may be replaced in many ways by different elements. If in this replacement NH 2 remains, the resultant substances NH 2 R are called amides , whilst the substitution products, NHR 2 , in which only NH remains, are called imides , and those in which none of the ammoniacal hydrogen remains, NR 3 , are known as nitrides . Free amidogen, N 2 H 4 , is now known in a state of hydration under the name of hydrazine; it combines with acids and resembles ammonia in this respect. In the action of different substances on ammonia it is the hydrogen that is substituted , whilst the nitrogen remains in the resultant compound, so to say, untouched. The same