The opening of XV. THE GROUPING OF THE ELEMENTS AND THE PERIODIC LAW
It is seen from the examples given in the preceding chapters that the sum of the data concerning the chemical transformations proper to the elements (for instance, with respect to the formation of acids, salts, and other compounds having definite properties) is insufficient for accurately determining the relationship of the elements, inasmuch as this may be many-sided. Thus, lithium and barium are in some respects analogous to sodium and potassium, and in others to magnesium and calcium. It is evident, therefore, that for a complete judgment it is necessary to have, not only qualitative, but also quantitative, exact and measurable, data. When a property can be measured it ceases to be vague, and becomes quantitative instead of merely qualitative.
Among these measurable properties of the elements, or of their corresponding compounds, are: (a) isomorphism, or the analogy of crystalline forms; and, connected with it, the power to form crystalline mixtures which are isomorphous; (b) the relation of the volumes of analogous compounds of the elements; (c) the composition of their saline compounds; and (d) the relation of the atomic weights of the elements.