Enthalpies of Combustion of the Pyridine N-Oxide Derivatives: 4-Methyl-, 3-Cyano-, 4-Cyano-, 3-Hydroxy-, 2-Carboxy-, 4-Carboxy-, and 3-Methyl-4-Nitro, and of the Pyridine Derivatives: 2-Carboxy-, and 4-Carboxy-. The Dissociation Enthalpies of the N-O Bonds