The present year represents the sesquicentennial of the real beginning of the science of metabolism. In 1777 the French chemist Lavoisier—on whom later the ax of the guillotine fell during the Reign of Terror—described the exchange of gases in respiration. This was the brilliant investigator who demonstrated that respiration in the body and combustion outside it are essentially analogous processes in representing an oxidative change with water and carbon dioxide as by-products. In a brief memoir published in 1777 he enunciated the principles that during respiration it was only "air eminently respirable" (oxygen) which disappeared from the atmosphere when an animal was put into a confined space, and that this air was supplanted by expired "aeriform calcic acid" (carbon dioxide); that when metals were calcined in air, oxygen was absorbed until its supply was exhausted; that after an animal had perished in a confined space and the carbon dioxide in