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and this heat could not then be converted to work. The implication of the Second Law was that all energy was continually running down or ‘dissipating’ into heat. Therefore the clockwork Universe must eventually run down unless there was something – or someone – outside the Universe to wind it back up again.

      The First Law showed that heat could not be indestructible and this led to the resurrection of an old theory that heat (and perhaps all forms of energy) were hidden forms of motion. In hot water, water molecules move around very rapidly, while in cold water the molecules move slowly: when hot and cold water are mixed, the rapidly moving molecules of the hot collide with the slow-moving molecules of the cold, slowing the rapid molecules and speeding up the slow molecules which results in lukewarm water. Thus, the transfer of heat is really a transfer of motion. The exchange between all types of physical force in a common currency of energy gave a great unity to late-nineteenth-century science; a unity missing in the eighteenth century when electricity, magnetism, heat, light and work were all different and discussed in different terms. In the nineteenth century, because these apparently different physical forms could be interconverted they came to be regarded as different forms or manifestations of one thing: energy. But energy was not a type of matter but rather the motion or arrangement of matter. This concept of energy gave a new boost to the hopes of mechanists, who thought they might finally be able to describe everything in the Universe in terms of matter in motion. It has been argued that the origin of this energy concept was partly due to new concepts in accountancy accompanying the rise of industrialization: it is certainly true that energy acted as a new currency within physics keeping track of mechanical transactions. Prior to the 1850s ‘energy’ did not exist as a useful concept in science, afterwards it became the central concept. However, the word ‘energy’ entered the English language in the sixteenth century, meaning roughly ‘vigour of expression’, and later ‘vigour of activity’. Originally the word was derived from Aristotle’s term energeia, meaning actuality/activity; this in turn is derived from the Greek en for in or at and ergon for work. Today the word ‘energy’ has a rather schizoid existence, meaning something technical and quantifiable to scientists, but having a variety of metaphorical meanings in the wider community.

      The scientific concept of energy did not arise purely from physics, but also at the same time from biology. Indeed the principle of energy conservation was simultaneously discovered by about twelve different scientists but was first formulated by the physicians Mayer and Helmholtz with reference to the forces of life. Robert Mayer (1814– 1878) was a German physician with an unlucky life. A mediocre student, he was arrested and expelled for joining a secret society. He eventually graduated and joined a ship bound for the East Indies as the ship’s doctor. At that time doctors still followed Hippocrates and Galen’s advice to bleed patients for a variety of maladies. While bleeding sailors in the East Indies, Mayer was alarmed to find that blood from the veins was much redder than usual, almost like arterial blood. At first, he worried he was puncturing arteries by mistake but local doctors assured him it was normal for venous blood to be redder in the tropics than in the cold north. This set Mayer thinking. He knew that Lavoisier had proposed respiration functioned to produce heat for the body and he also knew that the change from red to blue blood from arteries to veins was due to the removal of oxygen from the blood for respiration. Thus redder blood in the veins of a sailor in the tropics might be due to less respiration and heat production, which would make sense since the body needed to produce less heat in the tropics than the cold north. He also knew Lavoisier had shown men doing hard work respired more but had not given a convincing explanation of this important finding. Mayer proposed that fuel, heat and work were interconvertible: that it was possible to convert one into the others. Thus work done by men could be produced from heat (as in a steam engine) and this heat could in turn be produced by respiration (the burning of food). More work required more heat and more respiration as Lavoisier and Séguin had found experimentally. This reasoning, although partly wrong, was definitely getting closer to the secret of the energy of life.

      When Mayer got back to Germany he wrote up his ideas in a scientific paper, but his thinking was muddled and the paper was rejected. On a second attempt he sent the paper to von Liebig, who published it in 1842. However, when von Liebig published soon after a related theory, Mayer accused him of plagiarism. As Schwann would have agreed, it was not wise to oppose the powerful von Liebig. Mayer then got into even deeper water when he started a priority dispute with Joule as to who had first thought of the conservation of energy. But Mayer lost both arguments due to his unestablished position. The ‘Joule’ is now the standard scientific unit of energy and the ‘Kelvin’ the standard unit of temperature, while Mayer’s name is nowhere to be seen in the virtual world of scientific units. Understandably, he became depressed, suffering a mental breakdown and attempting suicide.

      Mayer’s ideas on the conservation of forces were not sufficiently general and quantitative to convince most scientists that something important had been discovered. This situation was dramatically changed by the great German physiologist Hermann von Helmholtz (1821–94), who in 1847 at twenty-six published his famous paper on the conservation of force. Helmholtz gave an exact quantitative definition of energy, explaining how the conservation of energy followed naturally from the known laws of physics. Using these principles, he suggested that heat and work generated by animals must derive entirely from the burning of food in respiration. Although Helmholtz was strongly sympathetic to von Liebig’s work, he pointed out that the vital force was incompatible with the conservation of energy (because the vital force could be converted into physical forces but not vice versa), and must thus be discarded by the new science of energy. Helmholtz was a founding member of a school of German physiologists (known variously as the Helmholtz, Berlin or 1847 School of Physiologists) who sought to explain all biological processes in terms of known physical, rather than vital, forces.

      According to Helmholtz’s version of the conservation of energy, there was a single, indestructible and infinitely transformable energy basic to all nature. This ‘Energy’ was more fundamental to the Universe than matter and force, as the overarching theory of the conservation of energy constrained the manifest forms of matter and motion. Energy was well on its way to replacing God. The good news of the First Law was that the Universe was now a vast reservoir of protean energy awaiting conversion into work. The bad news of the Second Law was that this conversion was taxed by the dissipation of some energy into heat. Although all forms of energy were equal, some forms were more equal than others.

      The discovery of the conservation of energy was partly due to the recognition that any quest to build a perpetual-motion machine was doomed. In the eighteenth century the French Academy of Sciences had set up a commission to examine proposals for building such a mythical machine: although many tried (including the young Mayer) all had failed. Such a machine would produce motion and work out of nothing. It would be an ‘unmoved mover’, something that Aristotle had associated with God alone. The recognition that perpetual motion was impossible led to the idea that all motion must arise from some prior, actual or potential motion: no change without a prior change. Therefore the whole history of the Universe was locked into one single causal web. Helmholtz criticized von Liebig’s concept of the vital force powering muscle contraction because the concept allowed the possibility of a perpetual motion machine which he considered impossible. But if energy conservation prevented the vital force from acting, some thought it would also prevent God interfering with the material world. Lord Kelvin magnanimously gave God a special dispensation to create or destroy energy. But others were less generous, relegating Him to the role of creating a fixed amount of energy at the start of the Universe and then sitting impotently on the sidelines as the consequences of His creation unfolded. Surprisingly some physicists now believe that the net amount of energy at the beginning of the Universe was zero, so perhaps it was God who was lacking in generosity.

      The ancient Greeks said Prometheus had stolen fire from the gods, given it to mankind and with it part of their divine knowledge and power. Now, through Helmholtz and the others, mankind had acquired the concept of energy itself, and with it a greatly increased power for good or evil. If this concept of energy could be used to understand the secret of life and death, then perhaps death itself could be conquered and humans might at last become immortal gods.

      The relation between respiratory heat production and muscle work and in general the coupling between

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