KESPIEATION 



298 



one 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 (fig. 131). 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 tem- 

 perature must be kept constant, 

 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 experi- 

 ment 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 ex- 

 posed to the sunlight during the daytime. If the caustic 

 potash is examined, it will be found to have gained con- 

 siderably in weight, and to contain a quantity of carbonate 

 of potassium, which must have been derived 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. 



