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This thesis describes an experimental and theoretical investigation of equilibrium properties and low-frequency wave phenomena in the positive column of a low-pressure arc in mercury-vapour. The structure of the positive column and the discharge kinetics were considered in detail. The conservation equations which govern then and the similarity laws which can be deduced therefrom were examined. The electron dynamics and in particular the ionization, mobility and energy processes were considered. The plasma sheath is of fundamental importance in determining the boundary conditions and mechanisms upon which many of the plasma phenomena depend. The properties of the plasma sheath in the actual conditions of a low-pressure arc were examined. Particular attention was paid to the generalization of Bohm's criterion in the presence of oscillations in the plasma as well as to the concept of sheath impedance and ways to measure this. Pressure effects in low-pressure arcs, which have been little studied and are little understood so far, are extensively analysed here and two distinct pressure effects are predicted. The experimental investigation of the waves and oscillations was carried out with the help of phase correlation and spectral analysis techniques. Ion-acoustic waves propagating along the column and uniform oscillations of the column as a whole were found to occur spontaneously in a wide range of discharge regimes. Ion-acoustic waves could also be impressed on the plasma column by a suitable exciter. The general properties of the ion-acoustic waves are reviewed, with special emphasis on the boundary conditions and propagating modes in a plasma column. The two-stream instability mechanism which operates in the actual conditions of a low-pressure arc is examined in detail. The properties of the observed ion-acoustic waves were found to be in general good agreement with the available theories, although a few points still require further investigation. In-phase and uniform oscillations have been reported to occur sometimes in plasmas. However, very little work exists in this particular field so far. The origin and nature of these oscillatory phenomena are examined are discussed. The specific case of a low-pressure positive column is considered. The theory of the purely radial ion-acoustic oscillation of a plasma column is given at first. Next, the electron temperature is allowed to vary as well, and the theory of the radial pulsation of a plasma column involving both electron temperature and density perturbations is developed. In the light of this theory, in-phase and even uniform oscillations of a plasma column are predicted. Moreover, when electron temperature and density perturbations coexist there is a net perturbation of the boundary losses experienced by the plasma column. This mechanism can give rise to the instability of the uniform oscillation of the column. It is believed that the observed large amplitude non-linear uniform oscillations of the positive column can be understood and identified in the light of this theory.