As a formal scientific discipline, biochemistry has emerged during the twentieth century. Some of the phenomena included within its modern domain have, however, been thought about and investigated for much longer. Within the Western scientific tradition the classic Greek writers, including the authors of the Hippocratic treatises, Aristotle, and Galen, gave much attention to digestion and nutrition, defined as the transformation of foodstuffs into the substance of the body. When chemistry emerged as a distinct field of operations and doctrines during the seventeenth century, theoretical applications were quickly made to explain these same physiological processes. The decisive demarcation of the beginning of the modern field of experimental investigation took place at the end of the eighteenth century when Antoine Lavoisier applied his quantitative methods of chemical analysis and his new knowledge of the composition of water, air, and carbon dioxide to make the first determinations of the elementary compositions of substances derived from plants and animals, and when he utilized the same methods, together with direct calorimetric experiments on animals, to demonstrate that respiration is a form of slow combustion. On the first of these foundations the field of organic chemistry grew to a flourishing enterprise by the 1830s. Gradually, however, organic chemists departed from their original preoccupation with biologically significant substances, and another fledgling field, known as physiological chemistry, appeared to fill this gap. For the rest of the century, however, physiological chemistry remained, with a few notable exceptions, an adjunct to physiology.