THE BIOMETER: HOW TO USE IT 133 
now left in the chamber A can be withdrawn through 
stopcock 4 into a receiving vessel. In order now to 
analyze the air in the tube B, it is better to clean the 
apparatus once more with water and dry it, as directed 
elsewhere. 
The chamber is now filled with mercury so that the 
remaining volume of it will be as little as possible, say 
15 c.c. (the exact volume need not be known here), 
the apparatus is sealed with mercury as usual, and then 
washed several times with air free of carbon dioxide, and 
then clear barium hydroxide is introduced into the usual 
tube inside of the chamber, forming a hemispherical 
drop at the top of d. If no deposit of barium carbonate 
forms on the surface of the drop within ten or fifteen 
minutes, we are sure that ordinarily the air we use is 
free from carbon dioxide and that the apparatus is in 
perfect condition. This point established, a small 
portion of the gas is driven from the tube B into this 
chamber A. This is done by withdrawing a desired 
amount of mercury from the chamber A into a receiving 
cylinder and adjusting the pressure in the chamber and 
tube B by means of mercury burette G. Close stopcock 
2 by turning it 45°. 
The surface of the drop at d should now be watched 
with a lens, as usual, for a deposit of carbonate. If no 
deposit appears within ten minutes, we should introduce 
more air from the tube, with usual care, until we get the 
first visible precipitate detectible with a lens during ten 
minutes’ standing. It is very important that we should 
give about ten minutes of time for the reaction after 
each withdrawal of the air from the tube B into the 
chamber A. 
