The Energy of Life: - Guy Brown

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and one days. He found that a small amount of solid matter appeared in the water but by weighing the matter, water and container demonstrated that all this matter was derived only from the container, thus proving water could not be transmuted into earth.

      Lavoisier next turned his attention to the burning of metals. Heating metals results in a rusting of the surface, which had been compared to combustion. But according to phlogiston theory (equating phlogiston with the element of fire) combustion results from the release of phlogiston from the material into the air, and should thus result in a decrease in weight of the remaining material. Lavoisier tested this by measuring the weight of the metal before and after heating. He found that the metal always gained weight after heating; and furthermore, part of the air around the metal disappeared after the heating. Thus, the phlogiston theory of metal combustion could not be correct: Lavoisier interpreted his findings to mean that during the heating of the metal, some of the air combined with the metal to form rust, thus increasing the weight of the metal. But what was it in air that combined with the metal?

      At this point (October 1774) Joseph Priestley visited Paris, dining with Lavoisier and other French scientists. This crucial meeting was to provide the essential key to Lavoisier’s research, but also resulted in the two scientists’ long-running, bitter dispute over scientific priority and plagiarism. Priestley (1733–1804) was a Presbyterian minister from Yorkshire who developed a surprising bent for science. While investigating the properties of carbon dioxide, derived from the brewery next door, Priestley discovered that when the gas was dissolved in water, it produced a pleasant drink (soda water, present in most soft drinks today). He received a prestigious medal from the Royal Society for this invention and was subsequently recruited by the Earl of Shelburne to be his secretary and resident intellectual. Priestley set up a laboratory at Shelburne’s country estate and proceeded to isolate a number of gases. In August 1774, Priestley first isolated oxygen by collecting the gas resulting from heating mercuric oxide. He found a candle burned more brightly and a mouse survived longer in a jar of this gas than in ordinary air. Priestley considered the new gas to be a variety of air (‘pure air’) and adhering to the phlogiston theory, later named it ‘dephlogisticated air’. At this crucial point Shelburne took Priestley to Paris and at a fateful dinner with Lavoisier, Priestley told of his recent experiments. Whether or not this meeting was the inspiration for Lavoisier’s subsequent experiments was later hotly disputed. But Lavoisier immediately repeated Priestley’s experiment of producing oxygen by heating mercuric oxide, realizing that this new gas must be the substance in air combining with the heated metal to produce rust (metal oxides). But Lavoisier interpreted the new gas as a separate substance (or element), not a variety of air, and later named it ‘Oxygen’ – which is Greek for ‘acid former’, because he believed (wrongly) that all acids contained some oxygen. In April 1775, Lavoisier presented his findings at the French Academy without reference to Priestley, claiming he had independently discovered oxygen. Priestley subsequently disputed his priority in the discovery of oxygen. There now seems little doubt that Priestley and Scheele discovered oxygen, but because they used the phlogiston theory and only had a crude conception of chemical elements, they failed to interpret their findings as a new substance.

      Another bitter dispute followed over the composition of water. Water was still regarded as an element, but Priestley, Cavendish and James Watt (famous for his discovery of the steam engine) had found that if a mixture of hydrogen and oxygen (or air containing oxygen) was ignited with a spark, then water was produced. They were, however, slow to publish their findings. An assistant of Cavendish visited Paris in 1783, innocently telling Lavoisier of their findings on the production of water from hydrogen and oxygen. Lavoisier immediately returned to the laboratory repeating the experiment, and went even further by reversing it; he heated steam to produce oxygen and hydrogen. He swiftly published the result, claiming priority for the discovery. This understandably caused a furore. But the important knowledge was that water was not, as previously thought, an element, but a combination of oxygen and ‘hydrogen’ (another name coined by Lavoisier, meaning ‘generator of water’). At last the four elements theory was falling apart and something had to take its place. Lavoisier provided that new system, essentially modern chemistry, according to which there are many elements, including oxygen, hydrogen, nitrogen, carbon and phosphorus, which can combine in various ways to produce compounds, which depending on their nature and conditions may be either solids, liquids, or gases.

      Lavoisier’s key contribution here was to accurately measure the change in weight and to use the principle of conservation of mass – the idea that regardless of what you do to an object it will not change in weight (as long as no mass escapes). Before Lavoisier’s breakthroughs it was not clear whether matter could appear or disappear during reaction or transformations. Lavoisier showed by weighing that the mass stayed the same during a reaction, and explicitly stated the principle of Conservation of Matter: matter could not be created or destroyed. He used this principle to track where the matter was going in a whole series of reactions. Because of Lavoisier’s principle, contemporary improvements in weighing techniques contributed to the development of chemistry, as much as the microscope contributed to biology. He also provided a nomenclature for chemicals, still in use today. All these changes amounted to a Scientific Revolution, which transformed alchemy into chemistry. The new system was rapidly adopted throughout Europe, only rejected by a few die-hard phlogiston theorists, including perhaps unsurprisingly, Priestley. There was no love lost between these two great scientists. Priestley, the experimentalist, regarded Lavoisier’s theories as flights of fancy; while Lavoisier, the theoretician, characterized Priestley’s investigations as ‘a fabric woven of experiments hardly interrupted by any reasoning’.

      Priestley moved to Birmingham in 1780 and joined the Lunar Society, an influential association of inventors and scientists including James Watt, Matthew Boulton, Josiah Wedgwood (engineer and pottery manufacturer), and Erasmus Darwin (poet, naturalist and grandfather of Charles). In 1791 Priestley’s chapel and house were sacked by a mob angered at his support for the French Revolution. He fled to London, and then, in 1794 at sixty-one, emigrated to America, settling in Pennsylvania, and becoming one of the New World’s first significant scientists.

      Lavoisier then teamed up with Pierre-Simon de Laplace, one of the greatest mathematicians in France. They wanted to investigate the relation between combustion and respiration. Combustion is the process of burning, usually accompanied by flame, such as the burning of a candle. Respiration had originally described breathing, but it had been discovered that this process was associated with the consumption of oxygen and production of carbon dioxide; ‘respiration’ thus came to stand for this process of gas exchange by organisms. Both combustion and respiration consumed oxygen from the air, replacing it with carbon dioxide and both produced heat. But could the conversion of oxygen to carbon dioxide by a living animal quantitatively account for all its heat production? In other words, was respiration really combustion, accounting for the heat produced by animals? They decided to compare the heat and carbon dioxide production of a respiring guinea pig and of burning charcoal (pure carbon). Lavoisier and Laplace invented a sensitive device to measure heat production, although it only worked well on days when the temperature was close to freezing. When, at last, everything was working, they found the burning of charcoal and the guinea pig’s respiration produced the same amount of heat for a given amount of carbon dioxide. They concluded therefore that the heat production of animal respiration was due to combustion of carbon (from food) within the animal, and that respiration was in fact slow combustion. From this result they had the audacity to claim that a vital living process was in fact a simple chemical reaction. And they were right – well, partly.

      Priestley had again been working on similar lines. He had shown that candles and mice lasted approximately five times longer in a jar of oxygen than in a jar of ordinary air. This is because ordinary air consists of one fifth oxygen and four fifths nitrogen, a gas which does not support life. Priestley said of oxgyen (or rather, as he called it, dephlogisticated air):

      ‘It is the ingredient in the atmospheric air that enables it to support combustion and animal life. By means of it most intense heat may be produced; and in the purest of it animals may live nearly five times as long as in an equal quantity of atmospheric air. In respiration part of this air, passing the membranes of the lungs, unites with the blood and imparts to it its florid colour, while the remainder, uniting with phlogiston

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