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Dmitrii Mendeleev, the Periodic Table 1 Primary Source 8.5 DMITRI MENDELEEV, THE PERIODIC LAW OF THE CHEMICAL ELEMENTS1 Dmitri Mendeleev (1834–1907) was a Russian chemist best known for his contributions to the study of chemical elements and particularly the discovery of the periodic law and the creation of the periodic table. Mendeleev was born in a Siberian village near Tobolsk, Russia, the youngest sibling in a large family. Mendeleev’s father passed away at an early age, but his mother devoted great efforts to ensure he received an excellent education. In 1866, Mendeleev became professor of chemistry in St. Petersburg where he developed theories regarding the elements. Mendeleev’s great law, which describes the relation of the qualities of elements to their atomic weights, was announced only a few years later. It divided the elements into related families, which enabled scientists to predict the discovery of many new elements. The text below, excerpted from Mendeleev’s Faraday Lecture presented before the Fellows of the Chemical Society in the Theater of the Royal Institution in London on June 4, 1889, discusses first the nature of modern scientific research and then his theory of periodic law. The lengthy text repays careful reading by presenting a lucid and direct example of insightful scientific thought. For a link to an abridged text, click here. For the original full text with complete scientific formulae, click here. The high honour bestowed by the Chemical Society2 in inviting me to pay a tribute to the world-famed name of Faraday3 by delivering this lecture has induced me to take for its subject the Periodic Law of the Elements this being a generalization in chemistry which has of late attracted much attention. While science is pursuing a steady and onward movement, it is convenient from time to time to cast a glance back on the route already traversed, and especially to consider the new conceptions which aim at discovering the general meaning of the stock of facts accumulated from day to day in our laboratories. Owing to the possession of laboratories, modern science now bears a new character, quite unknown, not only to antiquity, but even to the preceding century. Bacon’s4 and Descartes’5 idea of submitting the mechanism of science simultaneously to experiment and reasoning has been fully realized in the case of chemistry, it having not only been possible but always customary to experiment. Under the all-penetrating control of experiment, a new theory, even if crude, is quickly strengthened, provided it be founded on a sufficient basis; the asperities are removed, it is amended by degrees, and soon loses the phantom light of a shadowy form or of one founded on mere prejudice; it is able to lead to logical conclusions, and to submit to experimental proof. 1 Oliver Thatcher (ed.), The Library of Original Sources, 10 vols. (New York: University Research Extension, 1907), 10:254–261, 262–269. 2 The Royal Chemical Society in London was founded in 1841 and received a royal charter in 1847. 3 Michael Faraday (1791–1867) was an English scientist whose work revolved around electromagnetism. 4 Francis Bacon (1561–1626), an English philosopher who developed the theory of empiricism 5 René Descartes (1596–1650) invented analytic geometry and developed a philosophical basis for the mind- body dualism. 2 Willingly or not, in science we all must submit not to do what seems to us attractive from one point of view or from another, but to what represents an agreement between theory and experiment. Is it long since many refused to accept the generalizations involved in the law of Avogadro6 and Ampere,7 so widely extended by Gerhardt?8 We still may hear the voices of its opponents; they enjoy perfect freedom, but vainly will their voices rise so long as they do not use the language of demonstrated facts. The striking observations with the spectroscope which have permitted us to analyze the chemical constitution of distant worlds, seemed at first applicable to the task of determining the nature of the atoms themselves; but the working out of the idea in the laboratory soon demonstrated that the characters of spectra are determined, not directly by the atoms, but by the molecules into which the atoms are packed; and so it became evident that more verified facts must be collected before it will be possible to formulate new generalizations capable of taking their place beside those ordinary ones based upon the conception of simple substances and atoms. But as the shade of the leaves and roots of living plants, together with the relics of a decayed vegetation, favour the growth of the seedling and serve to promote its luxurious development, in like manner sound generalizations together with the relics of those which have proved to be untenable promote scientific productivity, and insure the luxurious growth of science under the influence of rays emanating from the centers of scientific energy. Such centers are scientific associations and societies. Before one of the oldest and most powerful of these I am about to take the liberty of passing in review the twenty years’ life of a generalization which is known under the name of periodic law. It was in March, 1869, that I ventured to lay before the then youthful Russian Chemical Society the ideas upon the same subject which I had expressed in my just written “Principles of Chemistry.” Without entering into details, I will give the conclusions I then arrived at in the very words I used: 1. The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties. 2. Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e. g., platinum, iridium, osmium) or which increase regularly (e. g., potassium, rubidium, caesium). 3. The arrangement of the elements, or of groups of elements, in the order of their atomic weights, corresponds to their so-called valencies,9 as well as, to some extent, to their distinctive chemical properties as is apparent, among other series, in that of lithium, beryllium, barium, carbon, nitrogen, oxygen, and iron. 4. The elements which are the most widely diffused have small atomic weights. 5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound. 6. We must expect the discovery of many yet unknown elements for example, elements analogous to aluminum and silicon, whose atomic weight would be between 65 and 75. 6 Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto, Count of Quaregna and Cerreto (1776– 1856) was an Italian scientist noted for his influence in molecular theory. 7 André-Marie Ampère (1775–1836) was a French physicist and mathematician and one of the founders of electromagnetism. 8 Charles Frédéric Gerhardt (1816–56) was a French chemist born in Strassbourg. 9 Valence (or valency) refers to the valence bonds an atom can form with other atoms. 3 7. The atomic weight of an element may sometimes be amended by a knowledge of those of the contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. 8. Certain characteristic properties of the elements can be foretold from their atomic weights. The aim of this communication will be fully attained if I succeed in drawing the attention of investigators to those relations which exist between the atomic weights of dissimilar elements, which, so far as I know, have hitherto been almost completely neglected. I believe that the solution of some of the most important problems of our science lies in researches of this kind. Today, twenty years after the above conclusions were formulated, they may still be considered as expressing the essence of the now well-known periodic law. Reverting to the epoch terminating with the sixties, it is proper to indicate three series of data without the knowledge of which the periodic law could not have been discovered, and which rendered its appearance natural and intelligible. In the first place, it was at that time that the numerical value of atomic weights became definitely known. Ten years earlier such knowledge did not exist, as may be gathered from the fact that in 1860 chemists from all parts of the world met at Karlsruhe10 in order to come to some agreements, if not with respect to views relating to atoms, at any rate as regards their definite representation. Many of those present probably remember how vain were the hopes of coming to an understanding, and how much ground was gained at that congress by the followers of the unitary theory so brilliantly represented by Cannizzaro.11 I vividly remember the impression produced by his speeches, which admitted of no compromise, and seemed to advocate truth itself, based on the conceptions of Avogadro, Gerhardt, and Regnault,12 which at that time were far from being generally recognized. And though no understanding could be arrived at, yet the objects of the meeting were attained, for the ideas of Cannizzaro proved, after a few years, to be the only ones which could stand criticism, and which represented an atom as “the smallest portion of an element which enters into a molecule of its compound.” Only such real atomic weights not conventional ones could afford a basis for generalization. It is sufficient, by way of example, to indicate the following cases in which the relation is seen at once and is perfectly clear: whereas with the equivalents then in use the consecutiveness of change in atomic weight, which with the true values is so evident, completely disappears.
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