118 A CHEMICAL SIGN OF LIFE 
the mercury in the little vessel can be sucked up to the 
,mark, thus making the remaining volume of the left 
chamber exactly 15 c.c.t_ With a pipette fill the mercury 
burette T with mercury to the mark, open stopcock G 
go° to the right, open stopcock C’ very gently till 
mercury falls to the second mark in burette 7, which is 
so marked that by introducing this amount of mercury 
the remaining volume in this chamber B is now 15 c.c. 
after the barium hydroxide is introduced to the top of 
the barium hydroxide tube in left chamber A. (There- 
fore, by introducing mercury to this mark, chamber 
B has a capacity less than 15 c.c., but by introducing 
barium hydroxide in the left chamber A, some of the 
mercury will be pushed back, so that the capacity of the 
right chamber B is now finally exactly 15 c.c.) Now 
shut stopcock C’ (very important). With a pipette add 
mercury to the mercury burette till the level of mercury 
in it becomes a little lower than that of the mercury in 
chamber B. Now pull out the core of stopcock J, so as 
1 The exact volume of each chamber should be calibrated once for 
all. If this is done, one can always work with a constant volume in 
both chambers, so that when a known amount in cubic centimeters of 
mercury is introduced so as to bring it up to the marks in the chambers 
the remaining volume will always be the same. The advantage in 
having both chambers equal in capacity is obvious. One of our appa- 
ratuses has a capacity of 21.5 c.c. in chamber A and 22 c.c. in chamber B. 
We therefore introduced 6.5 c.c. into A and marked the level of the 
mercury, and 7 c.c. into B and did likewise. Thus when mercury is 
introduced up to the marks, both chambers have the same remaining 
volume, namely, exactly 15 c.c. A little error in calibrating the chamber 
A is not very serious, as this chamber is used for the analytic purpose 
only, while the biometer is to be used as a quantitative apparatus; but 
the chamber B must be calibrated with extreme care, and each intro- 
duction of the known amount of the mercury must be done accurately. 
This can be accomplished by means of the mercury burette T, which is 
well calibrated and can measure off any known amount of mercury with 
a high degree of accuracy. 
