ABSORPTION OF GASES. 8 1 



ing solution of potash. The carbon dioxide exhaled by the branch intro- 

 duced through the cork A is absorbed by the potash. The water rises in 

 the lower part of the tube, indicating a diminution in volume of the air ; this 

 diminution is due to the absorption of oxygen by the branch. 



FIG. 1 6. A is a glass vessel containing a solution of baryta at its lower part ; a 

 branch is fixed in the cork, and the baryta solution is introduced through the 

 tubes B. As carbon dioxide is exhaled a precipitate of barium carbonate is 

 produced. 



In view of the large quantity of oxygen which, as we have 

 seen, is absorbed by plants, more especially by Fungi, we 

 must conclude that this gas is used up in their metabolic 

 processes, and with these we may also connect the exhalation 

 of carbon dioxide. The absorption of oxygen and the ex- 

 halation of carbon dioxide constitute what is known as the 

 Respiration of plants. The significance of this in the economy 

 of the plant will be discussed hereafter. 



Absorption of Carbon Dioxide. 



De la Hire (1690) and, after him, Bonnet (1754) found that 

 green plants or parts of plants when immersed in water and 

 exposed to sunlight gave off bubbles of gas ; and Bonnet 

 further observed that no bubbles were given off when 

 the water had been previously boiled. Priestley (1772) 

 pointed out that "fixed air" is absorbed by green plants 

 when exposed to sunlight, and shewed that the air evolved 

 by leaves under these circumstances is " dephlogisticated." 

 Senebier (1783) proved that the amount of " pure air" evolved 

 by green plants in water is greater when a considerable amount 

 of "fixed air" is held in solution, and he thus established 

 a connexion between the absorption of "fixed air" and the 

 exhalation of "pure air" by green plants when exposed to 

 sunlight. De Saussure (1804) confirmed Senebier's results, 

 and made some quantitative determinations of the amount of 

 carbon dioxide absorbed and of oxygen given off. 



He obtained, for instance, the following results : the stems and leaves 

 of seven Periwinkle plants (Vinca) were introduced into a glass receiver 

 which contained, in addition to the plants, air of the following composi- 

 tion : 



V. 6 



