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 T, opening the stopcock L, 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 i.oXio" 7 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 i . oX io~ 7 g., no matter how 

 large a space this amount of gas is occupying with the air. 



