Excerpt:
Dmitrii Mendeleev (1834-1907; see photo at Edgar Fahs Smith Collection, University of Pennsylvania) was born in Tobolsk, in Western Siberia. His chief contribution to chemistry was the establishment of the periodic system of elements. Mendeleev was one of a number of independent discoverers of the periodic law in the 1860s--that number ranging from one [Leicester 1948] to six [van Spronsen 1969] depending on the criteria one adopts. Mendeleev's formulation was clearly superior in several respects to the work of contemporary classifiers: it was the clearest, most consistent, and most systematic formulation, and Mendeleev made several testable predictions based on it. It was not, however, free from error. Scientists, even great scientists, trying to see further than others have in the past, do not always see the whole picture clearly. As noted below, Mendeleev himself corrected some of the errors within a few years; others persisted well into the 20th century.