This volume includes papers from 1887, when Lord Rayleigh became Professor of Natural Philosophy at the Royal Institution in London, to 1892. An 1888 contribution on the densities of hydrogen and oxygen led to a series of experiments on the densities of the atmospheric gases. This resulted in the unsettling discovery that the density of atmospheric nitrogen seemed very slightly to exceed the density of nitrogen derived from its chemical compounds. A substantial 1888 paper, on the wave theory of light, was written for the Encyclopaedia Britannica in the immediate aftermath of the crucial Michelson-Morley experiment in which the speed of light had been measured. In addition, this wide-ranging volume shows Rayleigh's developing interest in the properties of liquid surfaces, with a discourse on foams (1890), and a paper on surface films (1892). It also includes a charming brief appreciation (1890) of James Clerk Maxwell's legacy to science.