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. 



