In a previous paper two well-known crystalline compounds, sodiumpotassium tartrate, XaKC4H4O6. 4H2O, and sodium-ammonium tartrate, Na(NH4)C4H4O6.4H2O, were described, and it was therein shown how, in the crystals obtained by mixing these two isomorphous substances in any proportion, the dispersions of the ojjtic axes in the freshly prepared crystals were incompatible with orthorhombic symmetry. The crystals have been frequently measured and found to be orthorhombic bisphenoidal in external symmetry and this has been further confirmed by the author's measurements. A. full description of all the various dispersions to be found in the orthorhombic and monoclinic systems may be obtained from any text-book of crystallography, but in the following pages two of these are specially important.