THE BIOMETER: HOW TO USE IT 125 
cury from the left chamber A by opening the stopcock C 
and returning the same amount of mercury to the mer- 
cury burette 7, opening the stopcock LZ, and quickly 
shutting the stopcock, so that the communication of 
the gases between these chambers is momentary. This 
process of driving the known amount of the gas from the 
respiratory chamber to the analytic chamber must be 
done in a few seconds. The volume of mercury with- 
drawn from the analytic chamber is easily determined 
by drawing it into a small graduated cylinder, or, more 
accurately, by weighing it, and this volume corresponds 
to the exact amount of the gas we took from the right 
chamber to the left, since the pressures in A and B are 
kept exactly equal to atmospheric pressure during the 
transfer of the gas. 
One now watches the surface of the drop at d with a 
lens to see whether or not any deposit is formed during 
ten minutes. The presence or absence of any visible 
precipitate will decide whether the amount of gas 
taken from the respiratory chamber contained enough 
carbon dioxide to give a visible deposit. With this 
apparatus we have repeatedly introduced accurately 
known quantities of carbon dioxide of very high dilution 
into the left chamber and found with remarkable regu- 
larity that 1.01077 g. of carbon dioxide is the mini- 
mum amount which will cause a formation of detectible 
precipitate of barium carbonate during ten minutes. 
Smaller amounts of the gas than this will give no pre- 
cipitate for a long time, while larger amounts give it more 
quickly and it appears in larger quantities. There is a 
sharp line of demarkation at 1.0X 107’ g., no matter how 
large a space this amount of gas is occupying with the air. 
