But if all the animals of the world are continually consuming large amounts of oxygen, why doesn’t the oxygen in the atmosphere run out, as it does in the jar? Priestley discovered that plants produced large amounts of oxygen when a light was shone on them, and went on to suggest that all the oxygen used by the animals of the world is produced by plants. This suggestion is more or less correct, although the photosynthetic bacteria and algae of the sea (also now classified as plants) contribute as well to the production of oxygen, and it would take over two thousand years for the atmospheric oxygen to run out if all plants stopped producing oxygen. So both the food we eat and the oxygen we breathe come ultimately from plants; this means all energy is derived from plants, who in turn get their energy from the sun.
But if animal respiration was a type of combustion, where within the animal did it occur? Lavoisier and Laplace believed it happened in the lungs. They thought that carbon (and hydrogen) derived from food was brought to the lungs by the blood, and was burnt there with the breathed-in oxygen to produce the waste products of carbon dioxide (and water) then breathed out; and heat, which was absorbed by the blood and distributed to the rest of the body. Their belief that respiration was the combustion of food using oxygen was correct, but they were wrong in thinking that this combustion occurred in the lungs. Their view prevailed for fifty years, although Lagrange, the famous French mathematician, argued that the combustion could not occur solely in the lungs because if all the heat were released there they would be burnt to a cinder. He postulated that oxygen was taken up by the blood and the combustion of food occurred within the blood. This theory was very influential and competed with that of Lavoisier and Laplace. But in 1850, it was found that a frog muscle, separated from the body, still takes up oxygen liberating carbon dioxide; subsequently it was shown that the liver, kidneys, brain and all the body’s other tissues do the same. In the 1870s, the role of blood was demonstrated to be solely the transport of oxygen from the lungs to the tissues, where respiration occurred within the cells, the blood then carrying back the carbon dioxide generated to the lungs. The colour change of blood, from blue-back to red on passing through the lungs, was due to a single component of blood, haemoglobin, which picked up oxygen. Haemoglobin carried oxygen in the blood: it picked up oxygen in the lungs (changing from blue to red), then carried it to the tissues, where it released the oxygen (changing back from red to blue). Thus respiration (or combustion) was occurring not in the lungs but all over the body.
But it was still not clear what relations, if any, respiration and its associated heat production had to life and its processes such as movement, work and thinking. Lavoisier and Séguin, a co-worker, had shown (using Séguin as the experimental subject) that respiration increased during work, after a meal, in the cold, and in deep thought. Thus, there appeared to be a relation between respiration and physiological work, but it was hard to imagine how oxygen consumption or heat production could cause the movement of an arm, let alone the thinking of great thoughts. To bridge that conceptual gap required the imagining of something entirely new, and that something was ‘energy’.
THE VITAL FORCE
The collapse of the four elements theory opened up a cornucopia of matter. If ‘air’ was a mixture of different gases, ‘water’ was a combination of hydrogen and oxygen and ‘fire’ was not an element at all, then what on earth was ‘earth’? The science of chemistry, newly constituted and emboldened at the start of the nineteenth century, was salivating at the prospect of dividing ‘earth’ into thousands of different ‘species’. The concept of species and family had been successfully used by Linnaeus in the eighteenth century to bring order to biological taxonomy, but what were the building blocks of matter and how were they to be classified?
The theory of the elements was recast by Lavoisier, so that there were at least thirty different elements (now known to be about a hundred), existing as elementary, indivisible ‘atoms’ (proposed by Dalton in 1808) and combined in fixed ratios to form more or less stable ‘molecules’. Chemists divided their task between the analysis of inorganic and organic (or ‘organized’) matter, the latter being the constituents or products of living organisms. The alchemists had treated organic matter as if it were a single substance or a small number of elements, for example they had treated distillates of egg or urine as single substances. The chemists set about analysing the many components of egg and urine, using new methods of organic analysis. Lavoisier had pioneered such analysis by burning organic compounds in jars of oxygen and collecting the carbon as carbon dioxide and hydrogen as water. By quantifying the amount of carbon (C), hydrogen (H), and oxygen (O), a formula of the compound could now be written down; starch was, for example, thought to be C12H10O10. This formula was mistaken, and arose from the misconception that water was HO rather than H2O. But these methods were rapidly improved and applied with great enthusiasm by several German chemists, in particular Liebig and Wöhler. In 1835, Wöhler wrote: ‘Organic chemistry appears to me like a primeval forest of the tropics, full of the most remarkable things’. These first optimistic biological chemists did not, however, comprehend the full complexity and extent of their new field. It is now thought that there may be roughly five million different organic compounds in the human body and these compounds may be organized in an almost infinite number of different ways.
Nineteenth-century Germany, although not yet united, had become the major centre for scientific and technological innovation. Perhaps partly in reaction to the rise of science and industrialism, the Romantic movement developed in late-eighteenth-century Germany producing a scientific philosophy known as Naturphilosophie. This bizarre hybrid of Romantic philosophy and science contributed to a resurgence of interest in the vital force and the relationships between all forces.
Justus von Liebig (1803–1873) dominated German chemistry and biochemistry in the nineteenth century, sometimes to the detriment of biology. The son of a dealer in drugs, dyes, oils, and chemicals, von Liebig gained an interest in chemistry assisting his father. But he did badly at school and was derided when he suggested a career as a chemist. He learned to make explosives from a travelling entertainer, terminating an apprenticeship in pharmacy when he accidentally blew up the shop. His father packed him off to university to study chemistry but he was soon arrested and sent home after becoming too involved in student politics. Somehow he eventually earned his doctorate and went to work in Paris with one of the best French chemists of the time, Joseph Gay-Lussac. In the 1820s he took a position at a small German university at Giessen, and over the next twenty-five years produced a veritable mountain of chemical data.
However, von Liebig did not produce this data himself, rather he invented the research group as a quasi-industrial means of producing scientific results. Taking over an unused barracks as a chemical laboratory, he staffed it with junior scientists as lieutenants, students as foot soldiers and with himself as the distant but all-powerful general. This model of the research group was so successful in producing the large volumes of research required in the industrial world that it was widely adopted and remains the main means of producing scientific research today. This is in strong contrast to the pre-industrial system of the individual scientist thinking up experiments and carrying them out himself, with or without assistance. Von Liebig was both arrogant and argumentative and had a number of angry disputes with other scientists. His success gave him considerable power, through his control over scientific journals, appointments, and societies. The parallels with science today are unavoidable. It is dominated by a relatively small number of politicians of science who control the boards of scientific societies, journals, conferences, grant-giving bodies, and appointment boards. Success in a scientific career still depends to a certain degree on gaining the patronage of these politician-scientists.
Von Liebig started the prodigious task of analysing the millions of different combinations of elements – molecules – that make up a human being. Some kind of order was brought to this chaos by distinguishing three main types of molecule: carbohydrates, fats, and proteins. At first it was thought that these ‘organic’ molecules could only be produced by living organisms, using some kind of vital force. But in 1828 Friedrich Wöhler, a friend and colleague of von Liebig, found that he could chemically synthesize urea (an important component of urine) without any living processes being involved. Ultimately, this would lead to the melting of the boundary between the living and the non-living, but not yet.