from
Henry Fuseli () In 1773, the Priestleys moved to
Calne in
Wiltshire, southwestern England, and a year later
Lord Shelburne and Priestley took a tour of Europe. According to Priestley's close friend
Theophilus Lindsey, Priestley was "much improved by this view of mankind at large". Upon their return, Priestley easily fulfilled his duties as librarian and tutor. The workload was intentionally light, allowing him time to pursue his scientific investigations and theological interests. Priestley also became a political adviser to Shelburne, gathering information on parliamentary issues and serving as a liaison between Shelburne and the Dissenting and American interests. When the Priestleys' third son was born on 24 May 1777, they named him Henry at the lord's request.
Materialist philosopher Priestley wrote his most important philosophical works during his years with Lord Shelburne. In a series of major
metaphysical texts published between 1774 and 1780—''An Examination of Dr. Reid's Inquiry into the Human Mind
(1774), Hartley's Theory of the Human Mind on the Principle of the Association of Ideas
(1775), Disquisitions relating to Matter and Spirit (1777), The Doctrine of Philosophical Necessity Illustrated (1777), and Letters to a Philosophical Unbeliever'' (1780)—he argues for a philosophy that incorporates four concepts:
determinism,
materialism,
causation, and
necessitarianism. By studying the natural world, he argued, people would learn how to become more compassionate, happy, and prosperous. '', and Priestley was branded an
atheist. Priestley strongly suggested that there is no
mind-body duality, and put forth a materialist philosophy in these works; that is, one founded on the principle that everything in the universe is made of matter that we can perceive. He also contended that discussing the soul is impossible because it is made of a divine substance, and humanity cannot perceive the divine. Despite his separation of the divine from the mortal, this position shocked and angered many of his readers, who believed that such a duality was necessary for the
soul to exist. Responding to
Baron d'Holbach's
Système de la Nature (1770) and
David Hume's
Dialogues Concerning Natural Religion (1779) as well as the works of the French
philosophers, Priestley maintained that materialism and determinism could be reconciled with a belief in God. He criticised those whose faith was shaped by books and fashion, drawing an analogy between the scepticism of educated men and the credulity of the masses. Maintaining that humans had no
free will, Priestley argued that what he called "philosophical necessity" (akin to
absolute determinism) is consonant with Christianity, a position based on his understanding of the natural world. Like the rest of nature, man's mind is subject to the laws of causation, Priestley contended, but because a benevolent God created these laws, the world and the people in it will eventually be perfected. Evil is therefore only an imperfect understanding of the world. Although Priestley's philosophical work has been characterised as "audacious and original", it partakes of older philosophical traditions on the problems of free will, determinism, and materialism. For example, the 17th-century philosopher
Baruch Spinoza argued for absolute determinism and absolute materialism. Like Spinoza and Priestley,
Leibniz argued that human will was completely determined by natural laws; unlike them, Leibniz argued for a "parallel universe" of immaterial objects (such as human souls) so arranged by God that its outcomes agree exactly with those of the material universe. Leibniz and Priestley share an optimism that God has chosen the chain of events benevolently; however, Priestley believed that the events were leading to a glorious millennial conclusion,
Views on animals Priestley rejected
René Descartes's
animal machine doctrine, opposing the notion that animals are mere
automata without sensation or consciousness. Drawing on the
associationist psychology of
David Hartley, he held that mental functions arise from material processes and argued for strong continuity between human and animal minds. Priestley maintained that animals possess the same faculties as humans "in kind, though not in degree", attributing differences in behaviour to variations in brain structure rather than to the absence of soul or reason. In
Disquisitions Relating to Matter and Spirit (1777), Priestley criticised both Descartes and
Ralph Cudworth for attributing too little to animal cognition. He rejected Cudworth's concept of an immaterial "plastic nature" guiding animal behaviour, instead insisting that the minds of both humans and animals operate entirely through physical laws. Priestley also speculated that animals might participate in a future life, particularly in light of their capacity for suffering, asserting that divine justice must ultimately apply to all creatures capable of suffering. claiming that only the form of worship had been altered, not its substance, and attacking those who followed religion as a fashion. Priestley attended Lindsey's church regularly in the 1770s and occasionally preached there. He continued to support institutionalised Unitarianism for the rest of his life, writing several
Defenses of Unitarianism and encouraging the foundation of new Unitarian chapels throughout Britain and the United States.
Experiments and Observations on Different Kinds of Air Priestley's years in Calne were the only ones in his life dominated by scientific investigations; they were also the most scientifically fruitful. His experiments were almost entirely confined to "airs", and out of this work emerged his most important scientific texts: the six volumes of
Experiments and Observations on Different Kinds of Air (1774–86). These experiments helped repudiate the last vestiges of the
theory of four elements, which Priestley attempted to replace with his own variation of
phlogiston theory. According to that 18th-century theory, the combustion or
oxidation of a substance corresponded to the release of a material substance,
phlogiston. Priestley's work on "airs" is not easily classified. As historian of science
Simon Schaffer writes, it "has been seen as a branch of physics, or chemistry, or natural philosophy, or some highly idiosyncratic version of Priestley's own invention". Furthermore, the volumes were both a scientific and a political enterprise for Priestley, in which he argues that science could destroy "undue and usurped authority" and that government has "reason to tremble even at an air pump or an electrical machine". Volume I of
Experiments and Observations on Different Kinds of Air outlined several discoveries: "nitrous air" (
nitric oxide, NO); "vapor of spirit of salt", later called "acid air" or "marine acid air" (
anhydrous hydrochloric acid, HCl); "alkaline air" (
ammonia, NH3); "diminished" or "dephlogisticated nitrous air" (
nitrous oxide, N2O); and, most famously, "dephlogisticated air" (
oxygen, O2) as well as experimental findings that showed plants revitalised enclosed volumes of air, a discovery that would eventually lead to the discovery of
photosynthesis by
Jan Ingenhousz. Priestley also developed a "nitrous air test" to determine the "goodness of air". Using a
pneumatic trough, he would mix nitrous air with a test sample, over water or mercury, and measure the decrease in volume—the principle of
eudiometry. After a small history of the study of airs, he explained his own experiments in an open and sincere style. As an early biographer writes, "whatever he knows or thinks he tells: doubts, perplexities, blunders are set down with the most refreshing candour." Priestley also described his cheap and easy-to-assemble experimental apparatus; his colleagues therefore believed that they could easily reproduce his experiments. Faced with inconsistent experimental results, Priestley employed phlogiston theory. This led him to conclude that there were only three types of "air": "fixed", "alkaline", and "acid". Priestley dismissed the
burgeoning chemistry of his day. Instead, he focused on gases and "changes in their sensible properties", as had natural philosophers before him. He isolated
carbon monoxide (CO), but apparently did not realise that it was a separate "air".
Discovery of oxygen in Wiltshire, in which Priestley discovered oxygen|alt=Photograph of a laboratory, with glass-encased, wooden bookcases on two walls and a window on the third. There is a display case in the middle of the room. In August 1774 he isolated an "air" that appeared to be completely new, but he did not have an opportunity to pursue the matter because he was about to tour Europe with Shelburne. While in Paris, Priestley replicated the experiment for others, including French chemist
Antoine Lavoisier. After returning to Britain, he continued his experiments at
Bowood House and discovered "vitriolic acid air" (
sulphur dioxide, SO2). In March he wrote to several people regarding the new "air" that he had discovered in August. One of these letters was read aloud to the Royal Society, and a paper outlining the discovery, titled "An Account of further Discoveries in Air", was published in the Society's journal
Philosophical Transactions. Priestley called the new substance "dephlogisticated air", which he made in the famous experiment by
focusing the sun's rays on a sample of
mercuric oxide. He first tested it on mice, who surprised him by surviving quite a while entrapped with the air, and then on himself, writing that it was "five or six times better than common air for the purpose of respiration, inflammation, and, I believe, every other use of common atmospherical air". He had discovered
oxygen gas (O2). – who sympathised with
Unitarianism – built a laboratory for the famous dissenter at
Bowood House.|alt=Half-length portrait of a man wearing furred robes and a white wig and looking regal. Underneath his white robes, he is wearing red and gold and he is sitting in a red chair. Priestley assembled his oxygen paper and several others into a second volume of
Experiments and Observations on Air, published in 1776. He did not emphasise his discovery of "dephlogisticated air" (leaving it to Part III of the volume) but instead argued in the preface how important such discoveries were to rational religion. His paper narrated the discovery chronologically, relating the long delays between experiments and his initial puzzlements; thus, it is difficult to determine when exactly Priestley "discovered" oxygen. Such dating is significant as both Lavoisier and Swedish pharmacist
Carl Wilhelm Scheele have strong claims to the discovery of oxygen as well, Scheele having been the first to isolate the gas (although he published after Priestley) and Lavoisier having been the first to describe it as purified "air itself entire without alteration" (that is, the first to explain oxygen without phlogiston theory). In his paper "Observations on Respiration and the Use of the Blood", Priestley was the first to suggest a connection between blood and air, although he did so using
phlogiston theory. In typical Priestley fashion, he prefaced the paper with a history of the study of respiration. A year later, clearly influenced by Priestley, Lavoisier was also discussing respiration at the
Académie des sciences. Lavoisier's work began the long train of discovery that produced papers on oxygen respiration and culminated in the overthrow of phlogiston theory and the establishment of modern chemistry. Around 1779 Priestley and Shelburne – soon to be the
1st Marquess of Landsdowne – had a rupture, the precise reasons for which remain unclear. Shelburne blamed Priestley's health, while Priestley claimed Shelburne had no further use for him. Some contemporaries speculated that Priestley's outspokenness had hurt Shelburne's political career. Schofield argues that the most likely reason was Shelburne's recent marriage to Louisa Fitzpatrick: apparently, she did not like the Priestleys. Although Priestley considered moving to America, he eventually accepted
Birmingham New Meeting's offer to be their minister. Both Priestley and Shelburne's families upheld their Unitarian faith for generations. In December 2013, it was reported that
Sir Christopher Bullock—a descendant of Shelburne's brother,
Thomas Fitzmaurice—had married his wife,
Lady Bullock, née Barbara May Lupton, at London's Unitarian
Essex Church in 1917. Barbara Lupton was the second cousin of
Olive Middleton, née Lupton, the great-grandmother of
Catherine, Princess of Wales. In 1914, Olive and Noel Middleton had married at Leeds'
Mill Hill Chapel, which Priestley, as its minister, had once guided towards Unitarianism. ==Birmingham (1780–1791)==