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      Steam engines under construction at Boulton and Watt’s Soho Foundry, at Soho, near Birmingham, UK.

      Watt went on to apply science in many other areas of practical importance, including developing a process for bleaching cloth, and a successful early method for copying handwritten letters, a forerunner of the photocopier. In all of this work he provided the archetypal example of an experimenter/inventor, blurring the line between ‘pure’ science and ‘practical’ science. As Humphry Davy wrote of him: ‘Those who consider James Watt only as a great practical mechanic form a very erroneous idea of his character; he was equally distinguished as a natural philosopher and a chemist, and his inventions demonstrate his profound knowledge of those sciences, and that peculiar characteristic of genius, the union of them for practical application.’⁸

      In the early 1770s, Joseph Priestley, who was a non-conformist minister, philosopher and scientist, carried out some experiments that hinted at the importance of plants in making air fit to breathe. In 1771, while a minister in Leeds, he put some mint in a pot in a closed container glass with a lit candle. The candle soon went out, but the mint thrived and continued to grow. Twenty-seven days later, without having ever opened the container, he re-lit the candle by focusing sunlight through the glass of the container using a curved mirror. This showed him that the mint had somehow revived the air in the closed container. The following year, he carried out similar experiments with mice. First, he kept a mouse in a similar enclosed container with no plants, and noted how long it took before the mouse collapsed. Then, he repeated the experiment with living plants in the container with the mouse. This time, the mouse survived. Priestley realized that this meant that living plants provided something to the air that animals need in order to live, and that candles need in order to burn. At this time, however, he had no idea what the ‘something’ was.

      In 1774, Priestley left Leeds and was sponsored by Lord Shelburne, who provided him with a base on Shelburne’s estate in Calne, Wiltshire. Continuing his experiments there, he studied the gas released by what was then known as the red calx of mercury (now called mercuric oxide) when it was heated by focusing the rays of the Sun on it. He trapped the gas as it was given off, leaving mercury behind, and carried out a long series of experiments with it. He found that a lighted candle put into the gas flared up brightly, and that a glowing taper would re-light if plunged into a tube of this ‘pure air’, as he called it.

      In 1775, he did another mouse experiment. He put a full grown mouse in a container filled with ordinary air, and found that it could survive for only 15 minutes. But when he put a similar mouse in the same container filled with ‘pure air’, it survived for half an hour, and then, when he took the seemingly dead mouse out of the container and warmed it by the fire, it revived. News of these experiments was quickly spread via the Royal Society. Priestley had discovered oxygen, although it would not be given that name until later. A Swedish chemist, Carl Scheele, made the discovery at about the same time, but his results were not published until 1777.