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Monosymmetric, mon-ō-sim-et′rik, adj. having only one plane of symmetry in crystallisation.—adj.

The two forms, rhombic and monosymmetric, can exist in equilibrium at 95.6�, the transition point at which they have the same vapour pressure.

Thus rhombic sulphur when heated passes slowly at 95.6� C. into the 370 monosymmetric form which melts at 120�, but if heated rapidly the rhombic form melts at 114.5.

Gypsum thus crystallized is in its normal monosymmetric form, more stable under ordinary conditions than the orthorhombic form.

There is reason to suppose that the change described takes place in two stages, the gypsum first forming orthorhombic crystals and then crystallizing in the monosymmetric system.