I. In addition to the isomorphous series of mixed crystals studied in the first paper, and having the general composition {m NaK, n Na(NH₄)} C₄H₄O₆.4H₂O, the same three bases can unite to form the series {m KNa, nK(NH₄)}C₄H₄O₆·4H₂O and {m(NH₄Na, n(NH₄K}C₄H₄O₆. 4H₂O. Further, there are four other series found to be possible, in which only two of the bases NH₄, Na, and K are united with the acid. These are of the general type {mNa₂,nNa(NH₄)}C₄H₄O₆·4H₂O and { m(NH₄)₂,n(NH₄)Na }C₄H₄O₆·4H₂O; { mNa₂,nNaK }C₄H₄O₆·4H₂O and {mK₂,nKNa}C₄H₄O₆·4H₂O. All these series are isomorphous with each other, and are similar in habit and cleavage.

II. In the series which contain all three substituents NH₄, Na, and K the following effects are to be observed:

Initial crossed dispersion of the optic axial planes, with subsequent changes to a normal orthorhombic dispersion or to a crossed axial-plane dispersion of the brookite type.

III. In basal sections of the freshly prepared crystals, the greatest optical anomalies are exhibited by the terminal sections, and as the distance from the surface increases the anomalous effects fall off in magnitude. All sections, whether terminal or otherwise, ultimately possess the same optical properties when the crystals have reached the final state. This may be considered as evidence in favour of the homogeneity of these crystals.

IV. A rise in temperature hastens considerably in these crystals the process of settling down to the final state. Application of pressure, normal to the section, has a contrary effect. In neither case is there any effect on the final state when this is ultimately reached.

V. The strains involved degrade the symmetry of the freshly prepared crystals to that of a monoclinic class. Externally, however, judging from a large number of angular measurements, the crystals appear to be still orthorhombic holoaxial, and there is no corresponding readjustment of angular values as the settling down process in the interior takes place.