From under the Nose

This chapter looks at the elements in the final group of the periodic table—those elements known as the rare or noble gases. We shall see how their discovery in the atmosphere in the 1890s dates back to an observation first made by the meticulous Henry Cavendish over one hundred years earlier. This led to the unexpected discovery of an entire group of elements that needed to be added to the earliest periodic tables; and remarkably, one man was to dominate all these discoveries. One of Isaac Newton’s classic experiments was using a glass prism to split a beam of sunlight into a spectrum to show that white light is actually a mixture of all the colours of the rainbow. In 1802, William Hyde Wollaston (1766–1828), discoverer of the elements palladium and rhodium, modified the experiment by using a thin slit to admit the sunlight instead of the circular hole that Newton used. He subsequently discovered that the solar spectrum was not completely seamless, but actually contained a number of fine dark lines, now known as Fraunhofer lines. They get their name from Joseph Fraunhofer (1787–1826), who became the most skilled worker of glass and producer of lenses of the time. Using his highest-quality optical lenses, Fraunhofer observed that the solar spectrum had many dark lines; he mapped out over five hundred of these and designated the most distinct ones with the capitals letters A to H, with A and B being in the red region of the spectrum, and G and H in the violet. He used these as calibration lines in the development of better glasses for his optical instruments, and to demonstrate the superiority of his products compared with those of his competitors. The nature of the dark lines was not properly understood until the work of the German physicist Gustav Kirchhoff (1824–1997), who, in a beautiful collaboration with his colleague the chemist Robert Bunsen (1811–99), developed one of the most important analytical techniques still used in chemistry. It was with this technique that they discovered two new elements, and paved the way for others to discover many more.

Consolidation (1887–1895)

This chapter covers a period in which Boltzmann returned to the collision-based approach and consolidated it in answer to criticism and suggestions by William Thomson, Hendrik Lorentz, George Bryan, Gustav Kirchhoff, and Max Planck. He corrected errors in alleged counterexamples of equipartition by William Burnside and William Thomson; and in 1887, when the Dutch theorist Hendrik Lorentz detected an error in his earlier derivation of the H theorem for polyatomic gases, he devised a highly ingenious alternative. In 1894, he offered a new, simplified derivation of the Maxwell–Boltzmann distribution based on an idea by the British mathematician George Bryan. Together with Bryan, he also provided a kinetic-molecular model for the equalization of the temperatures of two contiguous gases. He denounced what he believed to be an error in Gustav Kirchhoff’s derivation of Maxwell’s distribution, and he strengthened Max Planck’s alternative derivation based on time reversal.