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Works by Dmitri Mendeleev
- Mendeléeff D. (1897) "The Grouping of the Elements and the Periodic Law". Chapter XV. In: Mendeléeff D. (1897) The Principles of Chemistry. Volume II. 6th edition. Translated from Russian by: George Kamensky. Edited by: T. A. Lawson. London: Longmans, Green and Co.
First paragraph: 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, hut also quantitative, exact and measurable, data. When a property can be measured it ceases to be vague, and becomes quantitative instead of merely qualitative.