RESPIRATION 



287 



to a bright sunlight, is shown in fig. 181. It consists of a 

 glass vessel which can be closed by a cork through which a 

 bent glass tube of small calibre is passed. The tube is carried 

 over and made to dip into a small dish containing mercury. 

 The bottom of the vessel is covered with finely broken glass, 

 upon which is poured 'a strong solution of caustic potash. 

 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 con- 

 stant, when the mercury will be 

 found to rise slowly and gradually 

 in the small glass tube, indicating 

 a diminution of the volume of the 

 air in the flask. If the experi- 

 ment is continued till the mercury 

 ceases to rise in the tube, and 

 the gas remaining in the vessel 

 is measured at the ordinary at- 

 mospheric pressure, and at the 

 temperature at which the expe- 

 riment 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 



FIG. 131. APPARATUS TO SHOW 

 THE ABSORPTION OF OXYGEN 

 BY A GREEN PLANT. 



