The line BC, representing the equilibrium between monoclinic and liquid sulphur, is **thermodynamically** calculable; the point B is found to correspond to 131° and 400 atmospheres.

Considered **thermodynamically**, voltaic cells must be divided into reversible and non-reversible systems. If the slow processes of diffusion be ignored, the Daniell cell already described may be taken as a type of a reversible cell.

- Since no gas is ideally perfect, it is most important for practical purposes to discuss the deviations of actual gases from the ideal state, and to consider how their properties may be **thermodynamically** explained and defined.

The slightest change in the load will cause motion in one direction or the other - the system is **thermodynamically** reversi ble.

By assuming suitable forms of the characteristic equation to represent the variations of the specific volume within certain limits of pressure and temperature, we may therefore with propriety deduce equations to represent the saturation-pressure, which will certainly be **thermodynamically** consistent, and will probably give correct numerical results within the assigned limits.

It is easy, however, to correct the formula for these deviations, and to make it **thermodynamically** consistent with the characteristic equation (13) by substituting the appropriate values of (v-w) and L =H -h from equations (13) and (is) in formula (21) before integrating.

The agreement of the values of H with those of Griffiths and Dieterici at low temperatures, and of the values of p with those of Regnault over the whole range, are a confirmation of the accuracy of the foregoing theory, and show that the behaviour of a vapour like steam may be represented by a series of **thermodynamically** consistent formulae, on the assumption that the limiting value of the specific heat is constant, and that the isothermals are generally similar in form to those of other gases and vapours at moderate pressures.