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now left in the chamber A can be withdrawn through 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 th'at 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 . 



