RESPIRATION 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- Fie. 131.—Arrararvs to snow 
‘ : THE ABSORPTION OF OxYGEN 
ment was started, it will be found ay 4 Gazew Pranv. 
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 durmg 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 
