A DEMONSTRATION OF PHOTOSYNTHESIS I09 
in /. The rubber tubing above E is squeezed to remove air. The 
meter stick is then inverted (without disturbing the tube J) and the 
rubber tubing at C is squeezed to remove air. When the air is all 
out of D the clamp at L is closed. The meter stick is returned to its 
upright position (as shown in the figure). The clamp L is then opened 
slightly so that when the rubber tubing above E is squeezed a move- 
ment of the liquid is seen in /. The clamp L, in this condition, per- 
mits the free movement of water while affording an effective barrier 
against the passage of a large gas-bubble. 
The tubes are now adjusted so that the surface of the liquid is at 
the same level in A as in /. The position of the bottom of the stopper 
B is read off on the meter stick (the spring clothes pins, 0, /, will hold 
it firmly in this position if properly adjusted). The clamp C is now 
opened somewhat, to secure atmospheric pressure in A, and the posi- 
tion of the meniscus at once read off on the meter stick (before lye has 
had time to diffuse into the tube above C). This gives the length of 
the gas column. 
The clamp C is now fully opened and moved out of the way and 
the meter stick inverted (without disturbing the tube J). In doing 
this the operator should grasp the meter stick at the ends and avoid 
touching the glass tubing. If the glass tubing is handled the gas may 
be warmed by the contact and the tubes may be pulled apart at the 
joints (since the rubber tubing becomes slippery from the lye). There 
is some strain on the joints at L and at the end of /; it is very impor- 
tant that they be firmly secured by winding with wire or string (or 
they may be firmly held by means of spring clothes pins). As these 
are permanent connections it may be advisable to heat the glass before 
slipping the rubber tubing over it. On cooling the rubber adheres 
firmly to the glass. There is little or no strain on the other joints, but 
they should be secured as a matter of precaution. 
When the meter stick is inverted the side neck of the T-tube should 
always point downward so as to prevent gas from entering it. 
The rubber tube G should be kept in such a position that it never 
kinks in such a manner as to prevent free passage of liquid. 
After inverting the meter stick several times in succession we 
restore it to the usual position and raise or lower the tube A until 
the liquid stands at the same level in A and in / (it may be necessary 
to pour more water into J). The length of the gas column in A is 
then read on the meter stick. The shrinkage, divided by the original 
length and multiplied by loo, gives the percent of CO2. 
