of H is replaced by (OH); that is, C 2 H 5 (OH). It is evident that the replacement of hydrogen by hydroxyl essentially forms the phenomenon of oxidation, because RH gives R(OH), or RHO. Hydrogen peroxide may in this sense be regarded as water in which the hydrogen is replaced by hydroxyl; H(OH) gives (OH) 2 or H 2 O 2 . The other form of substitution—namely, that of O in the place of H 2 —is also a common chemical phenomenon. Thus alcohol, C 2 H 6 O, or C 2 H 5 (OH), when oxidising in the air, gives acetic acid, C 2 H 4 O 2 , or C 2 H 3 O(OH), in which H 2 is replaced by O.
In the further course of this work we shall have occasion to refer to the law of substitution for explaining many chemical phenomena and relations.
We will now apply these conceptions to ammonia in order to see its relation to the oxygen compounds of nitrogen. It is evident that many substances should be obtainable from ammonia, NH3, or aqueous ammonia, NH4(OH), by substituting their hydrogen by hydroxyl, or H2 by oxygen. And such is the case. The two extreme cases of such substitution will be as follows: (1) One atom of H in NH3 is substituted by (OH), and NH2(OH) is produced. Such a substance, still containing much hydrogen, should have many of the properties of ammonia. It is known under the name of hydroxylamine,27 and, in fact, is capable, like ammonia, of giving salts with acids; for example, with hydrochloric acid, NH3(OH)Cl—which is a substance corresponding to sal-ammoniac, in which one atom of hydrogen is replaced by hydroxyl.28 (2) The other extreme case of substitution is that given by ammonium hydroxide, NH4(OH), when the whole of the hydrogen of the ammonium is replaced by oxygen; and, as ammonium contains 4 atoms of hydrogen, the highest oxygen compound should be NO2(OH), or NHO3, as we find to be really the case, for NHO3 is nitric acid, exhibiting the highest degree of oxidation of nitrogen.29 If instead of the two extreme aspects of substitution we take an intermediate one, we obtain the intermediate oxygen compounds of nitrogen. For instance, N(OH)3 is orthonitrous acid,30 to which corresponds nitrous acid, NO(OH), or NHO2, equal to N(OH)3 - H2O, and nitrous anhydride, N2O3 = 2N(OH)3 - 3H2O. Thus nitrogen gives a series of oxygen compounds, which we will proceed to describe. We will, however, first show by two examples that in the first place the passage of ammonia into the oxygen compounds of nitrogen up to nitric acid, as well as the converse preparation of ammonia (and consequently of the intermediate compounds also) from nitric acid, are reactions which proceed directly and easily under many circumstances, and in the second place that the above general principle of substitution gives the possibility of understanding many, at first sight unexpected and complex, relations and transformations, such as the preparation of hydronitrous acid, HN3. In nature the matter is complicated by a number of influences and circumstances, but in the law the relations are presented in their simplest aspect.
- It is easy to prove the possibility of the oxidation of ammonia into nitric acid by passing a mixture of ammonia and air over heated spongy platinum. This causes the oxidation of the ammonia, nitric acid being formed, which partially combines with the excess of ammonia.
The converse passage of nitric acid into ammonia is effected by the action of hydrogen at the moment of its evolution.31 Thus metallic aluminium, evolving hydrogen from a solution of caustic soda, is able to completely convert nitric acid added to the mixture (as a salt, because the alkali gives a salt with the nitric acid) into ammonia, NHO3 + 8H = NH3 + 3H2O.
In 1890 Curtius in Germany obtained a gaseous substance of the composition HN 3 (hydrogen trinitride), having the distinctive properties of an acid, and giving, like hydrochloric acid, salts; for