Cadmium sulphate, CdSO 4, is known in several hydrated forms; being deposited, on spontaneous evaporation of a concentrated aqueous solution, in the form of large monosymmetric crystals of composition 3CdSO 4.8H 2 O, whilst a boiling saturated solution, to which concentrated sulphuric acid has been added, deposits crystals of composition CdSO 4 4H 2 0.
The dynamical equilibrium between rhombic, liquid and monosymmetric sulphur has been worked out by H.
It forms monosymmetric crystals which by boiling with water yield amidosulphonic acid.
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.
Gypsum thus crystallized is in its normal monosymmetric form, more stable under ordinary conditions than the orthorhombic form.
Correlatively in its process of dehydration to form plaster of Paris, monosymmetric gypsum is converted into the orthorhombic form before it begins to be dehydrated.