If the substance in any state such as B were allowed to expand adiabatically (dH = o) down to the absolute zero, at which point it contains no heat and exerts no pressure, the whole of its available heat energy might theoretically be recovered in the form of external work, represented on the diagram by the whole area BAZcb under the adiabatic through the state-point B, bounded by the **isometric** Bb and the zero isopiestic bV.

Let BE be an **isometric** through B meeting AD in E, and EC an isopiestic through E meeting BC in C. Let BA, CD be adiabatics through B and C meeting the isothermal 0" in A and D.

It is generally convenient to divide the path into two steps, isothermal and **isometric**, or isothermal and isopiestic, and to integrate along each separately.

The intrinsic energy, E, is similarly represented by the area DZ'Vd under the adiabatic to the right of the **isometric** Dd.

The difference 90-E is represented by the area 9"DdO to the left of the **isometric** Dd under the isothermal B"D.

R is represented by the rectangle MDdO, bounded by the isopiestic and the **isometric** through D.

The felspathoid minerals, sodalite, haiiyne and nosean, which crystallize in **isometric** dodecahedra, are very frequent components of the phonolites; their crystals are often corroded or partly dissolved and their outlines may then be very irregular.