Considered from the standpoint of chemistry, living bodies appear to us as laboratories of chemical processes, for they undergo perpetual changes in their material substrate. They draw materials from the outside world and combine them with the mass of their liquid and solid parts.

Physiologie des Menschen (1830), Vol. 1, 'Allgemeine Betrachtungen der organischen Körper', 34. Trans. in Kenneth L. Caneva, Robert Mayer and the Conservation of Energy (1993)