Abstract

Summary James Watt (1736–1819) is best known as an engineer who dramatically improved the efficiency of the steam engine. What we take to be his chemical interests are conventionally seen as peripheral to his main line of work. He is usually treated as a chemist in three main contexts: his ‘practical’ chemical work relating to chlorine bleaching, varnishes, pottery, and so on; his work with Thomas Beddoes on the medicinal uses of various ‘airs’; his, much disputed, claim as a chemical discoverer in the case of the composition of water. In this paper, I argue that Watt himself, and his contemporaries, saw the centrepiece of his steam engine work—the separate condenser—as a chemical invention. I also suggest more broadly that Watt understood the steam engine as a chemical device. For Watt and his Scottish friends, the study of steam and heat was a chemical enquiry. The subsequent changes in the place of heat in chemical enquiry in the early nineteenth century led to a reclassification of Watt's chemical investigations as ‘physics’. This, in turn, produced the sharp separation of his chemical and engineering activities characteristic of modern historiography. Watt's steam engine, which is usually placed in the lineage of machines understood as heat engines, and explained by the laws of thermodynamics, is better seen in context as a chemical device. Watt's ‘indicator diagram’ is reassessed in the light of this.