RESPIRATION 287 
to a bright sunlight, is shown in fig. 1381. 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- 
i . i Fic. 181,—AppaRsaTus TO SHOW 
riment was started, it will be rae Axssorption or OxyGEN 
found that its volume has been *” 4 GR®ES PPANt- 
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 
