120 VEGETABLE PHYSIOLOGY 



Above the latter, supported by the glass so as not to be in 

 contact with the alkali, is placed the plant to be examined. 

 Watercress or any other herbaceous plant will answer 

 very well. The potash will absorb the carbon dioxide of 

 the atmosphere originally admitted as well as whatever 

 quantity of this gas is given off during the experiment. 

 As the experiment progresses the temperature must be 

 kept constant, when the mercury will be found slowly and 

 gradually to rise in the small glass tube, indicating a 

 diminution of the volume of the air in the flask. If the 

 experiment is continued till the mercury ceases to rise 

 in the tube, and the gas remaining in the vessel is 

 measured at the ordinary atmospheric pressure, and at 

 the temperature at which the experiment was started, it 

 will be found that its volume has been diminished by 

 about twenty per cent., and that what is left consists of 

 nitrogen. The oxygen will have been completely removed 

 by the green plant, even when the apparatus is left 

 exposed to the sunlight during the daytime. If the 

 caustic potash is examined, it will be found to have gained 

 considerably in weight, and to contain a quantity of car- 

 bonate of potassium, derived necessarily from the plant 

 during the experiment. The weight of this, will enable the 

 volume of the evolved carbon dioxide to be ascertained. 

 There will have been proceeding during the experiment an 

 absorption of oxygen, attended as before by an exhalation 

 of carbon dioxide, the latter having combined with the 

 potash. 



The evolution of carbon dioxide by the plant can be 

 more easily demonstrated by the use of the apparatus 

 shown in fig. 83. The jar A in the centre contains the 

 plant to be examined, which may preferably be represented 

 by a number of germinating peas. It is closed by a cork, 

 which is perforated in two places. Into one hole a tube is 

 inserted which passes to the bottom of the jar, and serves 

 for the admission of air. An outlet tube passes through 

 the other hole from the upper part of the jar, and leads to 



