[III] This part is concerned with the changes in bond order that take place in a conjugated system when the resonance integral of a bond, or the coulomb term of an atom is altered. It is shown that the sign of the mutual polarizability of two bonds in a chain depends upon whether there is an even or odd number of other bonds between them. Numerical values are given for mutual bond polarizabilities in some important hydrocarbons, and it is found that in the polyenes the effect of a perturbation decreases rapidly with distance along the chain. The effect of hetero-substitution on bond orders is considered, and the results compared with the predictions of the qualitative resonance theory. [IV] In this part it is shown that the force constant of a bond in a conjugated system depends upon its 'self-polarizability' as well as upon its mobile order, and that the interaction constant between a pair of bonds is closely related to their 'mutual polarizability'. The theory is applied to benzene, where good agreement is found with the scanty evidence available, and to naphthalene and butadiene, for which at present the force constants are not known.