49 2 Theory of the Nutrition [BOOK in.

revolution which was effected in chemistry between 1770 and 1790. It is a well-known fact 1 that this revolution dates from the discovery of oxygen-gas by Priestley in 1774. Priestley himself was and continued to be a stubborn adherent of the phlogiston ; but his discovery was made by Lavoisier the basis of an entirely new view of chemical processes. By the com- bustion of charcoal and the diamond, Lavoisier proved as early as 1776 that 'fixed air' was a compound of carbon and 'vital air.' In like manner phosphoric acid, sulphuric acid and, after a preliminary discovery by Cavendish, nitric acid also were found to be compounds of phosphorus, sulphur and nitrogen with vital air; in 1777 Lavoisier showed that fixed air and water are produced by the combustion of organic substances, and after establishing within certain limits the quantitative composition of fixed air, he named it carbonic acid, and the gas which had up to that time been known as vital air he called oxygen. Cavendish in 1783 obtained water by the combustion of hydrogen-gas, and then Lavoisier proved that water is a compound of hydrogen and oxygen. These discoveries not only did away step by step with the old theory of the phlo- giston, and supplied the principles of modern chemistry, but they also affected exactly those substances which play the most important part in the nutrition of plants ; every one of these discoveries in chemistry could at once be turned to account in physiology. In 1779 PRIESTLEY discovered that the green parts of plants occasionally exhale oxygen, and in the same year Ingen-Houss described some fuller investigations, which showed that this only takes place under the influence of light, and that the green parts of plants give off carbon dioxide in the dark, as those parts which are not green do both in the light and the dark. A correct interpretation of these facts was not however possible in 1779 ; it was not till ^7^5 that Lavoisier

1 See Kopp, ' Geschichte der Chemie' (1843), i. p. 306, and ' Entwick-

lung der Chemie in der neuerenzeit ' (1873), p. 138.