Liebig, JUSTUS, FREIHERR VON, chemist, was born at Darmstadt on 12th May 1803. The bent of his mind showed itself early. He studied chemistry at Bonn and Erlangen, and in 1822 went to Paris to perfect his studies. There he was introduced by A. von Humboldt to Gay-Lussac, who took him into his private laboratory, and along with him proved that the fulminates are identical in composition with the cyanates. Humboldt two years later secured for Liebig the appointment of professor of Chemistry at the university of Giessen. This chair he exchanged in 1852 for the corresponding one at Munich. He died on 18th April 1873. In 1845 he had been created Baron (Freiherr). Liebig was one of the most illustrious and fruitful chemists of his age, not less renowned for his investigations and discoveries in pure chemistry than for his researches in applied chemistry, and not less honoured for the reformation he effected in chemical method than for his highly important applications of chemical knowledge to the furtherance of the arts of life. As the inventor of the extract of beef and the prepared infant food, his name is known almost everywhere throughout the civilised world. He was the founder of agricultural chemistry, and thus the greatest reformer of practical agriculture in the 19th century. Closely connected with his work in this department were his researches into the nutrition of plants. He taught that each of the non-volatile saline ingredients found in the ash is essential to the life and growth of the plant, and that the plant gets them from the soil; this in course of time exhausts the soil and makes it barren, unless the elements which go to nourish the plant be resupplied to it, whether by means of manure or through the chemical action of the weather. Thus he directed attention to the cycle of transmutation between the mineral, the vegetable, and the animal kingdoms. In the department of animal physiology he made notable contributions to chemical science, demonstrating, amongst other things, that the heat of the animal body is wholly produced by the processes of internal combustion attendant upon the disintegration of nutritive matters; that different kinds of food serve different purposes in the body, and so admit of classification; that animal fat is produced within the animal organism from sugar and starch; and that spontaneous combustion in the human body is an impossibility. The phenomena of fermentation he explained as being purely chemical. He also investigated the constituents of the juices of flesh, and (along with Wöhler, q.v.) of uric acid, with most important results. This brings us to the region of pure organic chemistry. One of the most brilliant instances of the application of the methods of organic analysis in chemistry was Liebig's and Wöhler's discovery of the compound radicle benzoyl from the study of oil of bitter almonds and its derivatives. His investigations into the constituents of alcohol and its derivatives led him to oppose the existing view, that of the French chemists Dumas and Boullay, who regarded alcohol and ether as hydrates of olefiant gas; whereas Liebig denied the existence of the olefiant gas, and believed these compounds to be derivatives of a radicle ethyl, consisting of carbon and hydrogen. In the course of this inquiry he elicited for the first time chloroform and chloral; and it was whilst investigating the conversion of alcohol into acetic acid that he discovered the compound aldehyde. Then, by the clever use of the idea of the polybasic properties of certain acids, he succeeded in determining the constitution of organic acids. Among the practical discoveries and applications of Liebig may be mentioned the invention of silver-coated mirrors, an easy method for the preparation of potassic cyanide, now so largely used in electroplating, his plan for making unfermented bread, and his methods for analysing mineral waters.
When Liebig began to teach there were no public chemical laboratories in Germany. By his initiative one was established at Giessen; and from that have grown the admirably-equipped physical laboratories of the German and other universities. Besides stimulating the study of chemistry in this way, he vastly extended the use of the method of organic analysis, and invented such useful chemical apparatus as the appliances for analysis by combustion, the tube for determining molecular weight, and Liebig's condenser. His most important treatises, all translated into English, were Anleitung zur Analyse organischer Körper (1837); Die Chemie in ihrer Anwendung auf Agriculture und Physiologie (1840); Die Tierchemie (1842); Handbuch der organischen Chemie (1843); Chemische Untersuchungen über das Fleisch und seine Zubereitung zum Nahrungsmittel (1847); Die Grundzüge der Agriculturechemie (1855); Chemische Briefe (1844); besides numerous papers in scientific journals (317 in the Roy. Soc. Trans.). See works by A. W. Hofmann (1876) and W. A. Shenstone (1895).