The thesis presents an intellectual history of air. It investigates a critical period when the concept of air changed from being an all pervasive 'element' within a predominantly Aristotelian cosmology, to an 'ocean', or a fluid and particulate body with mass and weight. The thesis is set in the context of the seventeenth-century revolution in science in England and is focused on the pneumatic work of Robert Boyle. The question behind the thesis is raised by a specific experiment published by Boyle in 1672 when he describes how he tried, and failed, to produce gnats in a flask that had been evacuated of air by his air-pump. The historical aspect of the thesis examines the development of the vacuum, a new and revolutionary experimental site, in tandem with the equally revolutionary developments in physiology. The philosophical aspect of the thesis examines the conceptual ideas being played out in the Gnat Experiment and the relation between natural philosophy and theology. In terms of its empirical method the experiment was emblematic of the new science being developed by Boyle. The ambition behind the experiment, however, and Boyle's disappointment at its failure, engages another level of enquiry. Of particular interest is the problem of 'thinking matter' and the conflicts it provoked in relation to discussions of air and the vacuum, life and soul. Though reignited by Descartes, the discussion can be traced back to the early theories of air in Presocratic philosophy and the development of the 'pneumatic tradition' through later Socratic and Stoic philosophy, as well as Christian theology, in the guise of pneuma. It becomes apparent that Boyle's 'air' engages a complex field of concepts and arguments that can be traced back to the beginnings of philosophy and science, and that are still burning.