HCl, and a stable one, they are able to replace each other. And, indeed, we shall afterwards see (Chapter XI.) that in a number of instances a substitution between hydrogen and chlorine can take place. Given RH, then RCl is possible, because HCl exists and is stable. The molecule of water, H 2 O, may be divided in two ways, because it contains 3 atoms: into H and (HO) on the one hand, and into H 2 and O on the other. Consequently, being given RH, its substitution products will be R(HO) according to the first form, and R 2 O according to the second; being given RH 2 , its corresponding substitution products will be RH(OH), R(OH) 2 , RO, (RH) 2 O, &c. The group (OH) is the same hydroxyl or aqueous radicle which we have already mentioned in the third chapter as a component part of hydroxides and alkalis—for instance, Na(OH), Ca(OH) 2 , &c. It is evident, judging from H(HO) and HCl, that (OH) can be substituted by Cl, because both are replaceable by H; and this is of common occurrence in chemistry, because metallic chlorides—for example, NaCl and NH 4 Cl—correspond with hydroxides of the alkalis Na(OH) or NH 4 (OH). In hydrocarbons—for instance, C 2 H 6 —the hydrogen is replaceable by chlorine and by hydroxyl. Thus ordinary alcohol is C 2 H 6 , in which one atom 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, NH 3 , or aqueous ammonia, NH 4 (OH), by substituting their hydrogen by hydroxyl, or H 2 by oxygen. And such is the case. The two extreme cases of such substitution will be as follows: (1) One atom of H in NH 3 is substituted by (OH), and NH 2 (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 , and, in fact, is capable, like ammonia, of giving salts with acids; for example, with hydrochloric acid, NH 3 (OH)Cl—which is a substance corresponding to sal-ammoniac, in which one atom of hydrogen is replaced by hydroxyl. (2) The other extreme case of substitution is that given by ammonium hydroxide, NH 4 (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 NO 2 (OH), or NHO 3 , as we find to be really the case, for NHO 3 is nitric acid, exhibiting the highest degree of oxidation of nitrogen. 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, to which corresponds nitrous acid, NO(OH), or NHO 2 , equal to N(OH) 3 - H 2 O, and nitrous anhydride, N 2 O 3 = 2N(OH) 3 - 3H 2 O.