Apparatus For Estimating Carbon Dioxide 139 



placed on a glass plate 5 and weighed. The glass plate is hung on 

 n and m, which are electrodes fused into the side of the respiratory 

 chamber A. The chamber is now closed with the stopper R and 

 sealed with mercury. Through L, a connection is made with a 

 pump 6 and about 20 c.c. of mercury is introduced through G. Not 

 too much mercury should be used; its surface should not be within 

 5 mm. of the cup F. Then wash the whole apparatus with carbon 

 dioxide-free air, 7 which is introduced through C, by successive 

 evacuations. After the evacuation and washing out with pure air, 

 which is repeated three or four times, the pressure inside of the bulbs 

 is made equal to the atmospheric pressure by adjusting it at the nitro- 

 meter in the usual fashion. Stop-cock E is then closed, and the 

 space between E and L is evacuated so that the barium hydroxide 

 can rush in, a process which is very advantageous to obtain a clear 

 barium hydroxide solution. Then clear barium hydroxide solution 

 is run in through L. By opening E very slowly and carefully, the solu- 

 tion is now introduced into the chamber so that a small drop stands 

 up upon the upturned end of the capillary at F. Then the connection 

 between the two chambers is closed by D. It is imperative that this 

 drop of the solution should be perfectly clear at the start. If no 

 deposit of barium carbonate forms on the surface of the drop within 

 ten minutes, 8 a portion of the sample gas is drawn into B by with- 

 drawing mercury through G and opening the stop-cock D. The 

 volume of mercury withdrawn, which may be readily determined by 

 volume, or more accurately by weight, gives the volume of the sample 



5 The kind of glass plate used in connection with the nerve and small animals 

 like Planaria is shown on p. 120, Fig. i. (The first paper.) 



6 The pump should be capable of giving a vacuum of at least 25 or 30 mm. of 

 mercury. 



7 Air cannot be freed completely from carbon dioxide by passing it through 

 wash bottles. In my work, carbon dioxide-free air is prepared by shaking air with 

 twenty per cent solution of sodium hydroxide in a tightly-stoppered carboy, fitted 

 with suitable tubes. When this is to be used, it is driven into a nitrometer which 

 is filled with less concentrated alkaline solution (a weak solution is used so that 

 the chamber may not be too dry) by displacing it by running in a solution of 

 sodium hydroxide. After each evacuation, this air is introduced from the nitro- 

 meter into the chamber A through stop-cock C. 



8 If no precipitate appears within ten minutes, it is a sure control that the 

 apparatus is free from carbon dioxide. 



