Among the properties measured for each new element was the atomic weight or, for those investigators agnostic with respect to atoms, the equivalent weight. In the 50 years after Dalton's atomic hypothesis, the determination of atomic weights suffered from uncertainties in chemical formulas. The development of new tools for chemical analysis and more accurate techniques could determine with ever more precision the composition of compounds in terms of mass ratios. But what those masses represented remained unclear. For example, magnesium oxide was 60% magnesium and 40% oxygen by weight; but how many atoms of each element do those ratios represent?
More than 100 of Europe's most prominent chemists gathered in Karlsruhe in 1860 to try to reach some agreement about atomic weights, nomenclature, and even such fundamental terms as atom, molecule, equivalent. [Wurtz] The Congress adjourned without reaching any definite agreements. Yet the views of Stanislao Cannizzaro, disseminated to congress participants in a pamphlet, won near universal acceptance within a very few years. The pamphlet [Cannizzaro 1858] adopted Avogadro's hypothesis [Avogadro 1811]; it removed the objections the hypothesis had originally faced, and sketched a self-consistent system based on it.
With a set of atomic weights based on Cannizzaro's system, chemists had a scale according to which the elements could be placed in order. When placed in this order, properties of the elements recurred at various intervals. By the end of the 1860s, the first periodic classifications of the elements were proposed.
The selections of this section all pertain to the periodic system of elements. The work of J. A. R. Newlands shows the intellectual groping of a mind that recognizes the periodicity of elementary properties, but cannot quite formulate a self-consistent classification based on that periodicity. Dmitrii Mendeleev proposed the best known and most useful periodic system of the 1860s. The abstract in which that system first appeared in Western Europe and a review by Mendeleev of the periodic law 20 years later are included in this section. The last two selections treat the discovery of an element, argon, which had no place in the periodic table of the time and the effort to incorporate it into the periodic system.
The history of the periodic system is a fascinating one, full of priority disputes and polemics. Presenting that history in detail is not the purpose of the present section. For those interested in that history, I recommend Scerri 2006 and van Spronsen 1969 and the multitudinous references therein. Van Spronsen names six co-discoverers of the periodic law: Alexandre Émile Béguyer de Chancourtois de Chancourtois, Detlef Hinrichs, Julius Lothar Meyer, and William Odling in addition to Mendeleev and Newlands. I am not sure I agree with the claims for all six; however, van Spronsen provides much detail to allow the reader to arrive at his or her own conclusion.