This chapter analyses changing perceptions of blood in eighteenth-century medicine. Chemical analyses of the peculiar properties of blood led to the development of new understandings of physiology. But this approach developed by Herman Boerhaave and Hieronymus Gaubius also provoked criticism: Thomas Schwencke grew deeply sceptical about chemistry, convinced as he was that blood in vitro and blood in vivo were drastically different fluids. Coining his method ‘haematology’, Schwencke preferred measuring the weight and temperature of blood for diagnostic purposes. Analysing these competing claims, Verwaal argues that the debate went beyond the problem of methodology, and was directly linked to the essential question: was blood alive? Verwaal, then, offers a new perspective on perceptions of blood and the living body in the eighteenth century.