Protein chemistry began in the mid-eighteenth century with investigators who extracted proteins from vegetable and animal sources. The study of animal and vegetable substances was a subject of increasing activity throughout the eighteenth century. Food shortages and high prices caused by the wars between France and England and the political upheavals at the end of the eighteenth and beginning of the nineteenth century led to the study of the chemical constituents of plants and animals used for human food. In 1816 Magendie investigated the source of organic nitrogen in the animal—was it the air or food? He discovered from feeding experiments that nitrogenous foods are needed for life. Dumas used metallic copper for reduction of nitrogen oxides to elemental nitrogen after first activating the copper by heating in a stream of hydrogen. The J. Kjeldahl method for organic nitrogen was very important for the development of organic chemistry during the closing years of the nineteenth century.