A selective history of the benzene problem is presented, starting with August Kekulé's proposal of a hexagonal structure in 1865 and his hypothesis of 1872 that the carbon–carbon bonds oscillate between single and double. Only those theories are included that were accepted or at least discussed by a significant number of chemists. Special attention is given to predictions, their empirical tests, and the effect of the outcomes of those tests on the reception of the theories. At the end of the (...) period covered by this article, chemists generally accepted the valence bond theory proposed by Linus Pauling; some of them considered this a more sophisticated version of Kekulé's oscillation hypothesis. The sequel describes the replacement of the valence bond theory by the molecular orbital theory in the period ending around 1980. (shrink)

By a close examination of changes in analytical chemistry between the years 1920 and 1950, I document the case that natural science has undergone and continues to undergo a major revolution. The central feature of this transformation is the rise in importance of scientific instrumentation. Prior to 1920, analytical chemists determined the chemical constitution of some unknown by treating it with a series of known compounds and observing the kind of reactions it underwent. After 1950, analytical chemists determined the chemical (...) constitution of an unknown by using a variety of instruments which allow one to discriminate chemicals in terms of their physical properties. This transformation involved changes in the practice of analytical chemistry. It involved the development of a new family of scientific instrument-making companies, and a new level of capital expenditure necessary to do analytical chemistry. It involved the development of new means to disseminate information about scientific instruments. While I do not document the broader case, these changes have been widespread throughout the natural sciences. It is the rise in the importance of instrumentation which is the key conceptual change in what has been identified as the fourth big scientific revolution. (shrink)