Historians of chemistry usually associate the eighteenth century with the Chemical Revolution, but it could just as readily be called 'the century of gases' (or 'airs', as they were called in the eighteenth century). In the early part of the century, the British pneumatic chemists struggled to replace the traditional notion 'Air', understood as an inert chemical element, with the concept of 'air', regarded as the third state of matter, encompassing a wide variety of chemical species. These developments constituted a necessary condition for the Chemical Revolution, which occurred in the latter part of the century. In 'Observations', Priestley took pneumatic chemistry to a new level, with the discovery of eight simple inorganic gases. Motivated by his belief in a benevolent God and a pious utilitarianism, Priestly explored the role of the atmosphere in the balance of nature and the politics of the state, which he linked to the movement of Rational Dissent. He styled himself an 'aerial philosopher' to signal the interdisciplinary nature of his inquiries, which he regarded not as a branch of ordinary chemistry, but as a mode of thought that encompassed physics, chemistry and natural theology. Priestley saw it as a source of principles and secrets of nature more extensive than that of 'gravity itself'. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.
Keywords: aerial philosopher; air(s); phlogiston; pneumatic chemistry.