Three illustrations of the law of change of the crystal properties with variation of the atomic weight of the determinative elements of an isomorphous series may be given, and will serve to render the practical meaning of the generalisation clearer. The first is a diagrammatic representation, in Fig. 60 (in a very exaggerated manner as the real change would be inappreciable on the scale drawn), of the change of angle on replacing the potassium in potassium sulphate, K_{2}SO_{4}, or selenate, K_{2}SeO_{4}, by rubidium or cæsium. The inner crystal outline, a vertical section, is that of the potassium salt. The vertical lines represent the intersections of the two faces of the brachypinakoid _b_ = {010} with a vertical plane parallel to the macropinakoid _a_ = {100}; the horizontal lines represent the intersection of the two faces of the basal plane _c_ = {001} with the same vertical plane; and the oblique lines represent the intersection of the vertical plane with the four faces of the dome form _q_ = {011}, which are inclined to both _b_ and _c_ planes. The diagram is thus designed to show the variation of the inclination of these latter dome faces to the two rectangular axial plane faces _b_ and _c_. The outer crystal outline represents a similar section of a crystal of the corresponding cæsium salt, and the middle outline that of a crystal of the rubidium salt. The progressive alteration of the angle of the q-face will be obvious, the direction of the change being correct, but the amount of change, as already stated, being much exaggerated; in reality it never reaches a degree between the two extreme (potassium and cæsium) salts. It will be remembered that the respective atomic weights of potassium, rubidium, and cæsium are 38·85, 84·9 and 131·9, when hydrogen equals 1, that of rubidium being almost exactly the mean. [Illustration: FIG. 61.—Diagram illustrating Progressive Change of Double Retraction in Isomorphous Series. ]