A biography of Henry Edward Armstrong, an underappreciated maverick in the history of chemistry.  Fifth Business is a biography of the English chemist, educator, and scientific critic Henry Edward Armstrong. Today, Armstrong, who was a central figure in the development of the science of chemistry between 1885 and 1914, is more remembered for his campaigns to improve the teaching of chemistry, and science generally, and less for his theory of residual affinity and reverse electrolysis—or his hostility toward physical chemistry. However, right up until his retirement, Armstrong was a significant and prolific organic chemist, as well as a major figure in the academic and social life of the Chemical Society.   Fifth Business is structured as chronologically as possible, with Armstrong’s life and achievements as an active chemist in Part I (1848–1911) and as a critic in his long retirement in Part II (1911–1937). Brock’s authoritative biography provides a unique inside look at its subject, allowing us to better understand the history of British science, scientific institutions, scientific education, pedagogical theory, and social relations of science during the last third of the nineteenth century and the first third of the twentieth.