no 
W. J. V. OSTERHOUT 
After the reading is taken we invert the meter stick several times 
and take another reading. This should be continued until two suc- 
cessive readings agree. 
We now open the clamp at E, allowing the pyrogallol to enter 
and absorb the oxygen. The procedure is the same as for CO2. 
The shrinkage in the length of the column, due to absorption by 
pyrogallol, divided by the original length of the column and multiplied 
by 100 gives the percent of O2. 
If the apparatus were closed during the absorption by KOH and 
by pyrogallol there would be a tendency to suck in air at the joints. 
This is prevented by maintaining a passage for water through the 
clamp L, which is kept slightly open for this purpose. 
It may be added that all the tubes (whether glass or rubber) must 
be large enough to permit gas to pass freely into them and displace 
water, and that / must be large enough to be easily filled with liquid 
through the thistle tube. An internal diameter of 10 mm. will be 
found sufficient. 
In order to clean the apparatus it should be placed in running 
water under the tap and disconnected at C and E. 
A slender tube of metal or glass should be connected to the tap 
and inserted to the very bottom of ^ , so as to rinse it thoroughly. 
Students should practice analyzing the laboratory air for oxygen 
until the results are correct to within at least 2 percent. If the gas 
column is 600 mm. long 2 percent is 12 mm. on the tube. It is easy 
to read to i or 2 mm. (parallax must be avoided) . 
Since all the work is done at laboratory temperature (the reagents 
being at the same temperature) no correction is needed. 
As we now know the composition of the gas at the start, we may 
expose the plant to sunshine (for several days in succession if desired) 
and again analyze the gas by removing another of the long tubes. 
If photosynthesis has not progressed satisfactorily another exposure 
may be made and the gas subsequently analyzed by removing another 
tube. After all the tubes are removed new tubes (filled with air or 
with water) may be attached, a correction being made for the air or 
water thus added to the system. 
When the plant is exposed to sunshine the gas is heated and tends 
to escape at the joints: on cooling air may be sucked in. To prevent 
this (and to prevent diffusion of CO2 through the rubber) all the 
rubber tubing should be coated with paraffin. 
