July, 1893. PLANT NUTRITION. 19 



sulphur dioxide, as well as the influence of light and heat. All these 

 seemed to yield no definite results, but ultimately the action of plants 

 was found to be the chief cause. Mice were allowed to die in a closed 

 glass vessel, and the confined air was then divided into two parts, and 

 enclosed in glass vessels placed upside down over water, so that no 

 air could enter or escape. Into one, sprigs of mint were introduced, 

 and after eight days a mouse placed in it lived perfectly well, whereas, 

 in the other vessel not so treated, mice died almost instantly. 



By many experiments extending over two or three years, the 

 reversal of the effects of animal respiration by plants was established 

 and also the discovery made that green aquatic plants placed in 

 sunlight in water containing " fixed air " (carbon dioxide), give off 

 " dephlogisticated air " (oxygen). When the evolution of oxygen 

 stopped, the introduction of fresh-water was folloAved by a brisk 

 development of more bubbles, thus supporting the view that the 

 oxygen came from the "fixed air" dissolved in the water, and not 

 from the leaf. 



Ingenhousz, in 1779, confirmed and extended Priestley's obser- 

 vations. The latter had suggested that the main purifying effects 

 upon the air were due to the mere growth of the plant, but Ingenhousz 

 conclusively proved that the influence of the sun's light upon the 

 plant was an essential factor ; in darkness, although kept at the same 

 temperature as during the daytime, no purification took place, but 

 rather the reverse. 



Senebier, working independently about the same time as 

 Ingenhousz, went over the same ground and laid stress upon the 

 fact that only the green leaves give off oxygen in sunlight, and also 

 stated more precisely that the amount of oxygen evolved was 

 dependent upon the amount of carbon dioxide present in the water 

 used, distilled water yielding practically no gas at all. 



The observations, so far, had been chiefly qualitative in character, 

 and it was not until 1804 that De Saussure's quantitative investiga- 

 tions were published. 



By this time Lavoisier's famous researches had given an intelli- 

 gible account of the processes of combustion and respiration — the 

 composition of carbon dioxide was ascertained and the part played 

 by oxygen in producing it was fully understood. The more extended 

 use of the balance had also asserted the indestructibility of matter, 

 and under the new regime progress became possible in all branches 

 of science. 



De Saussure, taking advantage of the facts and spirit of this new 

 chemistry, introduced accuracy and terse completeness into his work. 

 Various species of plants were placed in air of known volume and 

 composition, and after exposure to sunlight for thirty-six hours the 

 air was examined and found to have been altered in composition. The 

 volume remained the same, but the carbon dioxide had disappeared 

 and a nearly equal amount of oxygen had been substituted for it. 



C2 



