Xo. 630] SHORTER ARTICLES AND DISCUSSION 95 



white background. It should be the same in all. The first flask 

 is kept as a control, nothing being added. It is inverted, and 

 the cork with its test tube containing the indicator solution re- 

 moved and quickly stoppered with a paraffined cork. To the 

 second flask one drop of the carbon dioxide saturated water is 

 added, to the second two drops, to the third four, or any desired 

 number, etc. The stopper in each case is quickly replaced and 

 the whole apparatus shaken vigorously until the CO, has dis- 

 tributed itself between the solution and the air. The test tubes 

 are then removed and corked as described above. The amount 

 of C0 2 added to each c.c. of the solution may now readily be cal- 

 culated by taking into account the absorbtion coefficient for the 

 temperature in question and the relative amounts of water and 

 air. The volume of a single drop of the added solution has, of 

 course, previously been determined by counting the number of 

 drops required to give a volume of, for example, 5 c.c. In add- 

 ing the C0 2 it is convenient to use a fine-pointed pipette which 

 can be inserted in the escape tube of the test tube in which the 

 water has been charged. This makes it unnecessary to remove 

 the stopper of the latter or otherwise to disturb it. Unless the 

 pipette is first filled with C0 2 (which may, however, readily be 

 done and which is to be recommended), a little of the gas will 

 escape from the free surface of the liquid within the pipette. If 

 only a few drops are used from the lower portion of the pipette, 

 however, no error will result if one works quickly enough. To 

 drop the solution into the flasks without allowing any appreciable 

 amounts of C0 2 to escape into the air requires a little practice, 

 but after a few trials the best method is discovered and the nec- 

 essary skill acquired. 



When the standards have been prepared, which requires only 

 a few minutes at the most, everything is ready for an actual 

 measurement. The procedure is as follows. After filling the 

 prepared test tube with out-of-door air, 1 c.c. of the indicator 

 solution is placed in its lower portion and the organism to be 

 studied in its upper portion, either free or attached to the cork 

 by a loop of thread. The tube is closed and agitated gently, 

 either continuously or at intervals, to mix the air and the indi- 

 cator solution thoroughly. It is neither desirable nor necessary 

 to shake the tube vigorously ; with a little practise it will be dis- 

 covered that a very slight movement of the proper sort will keep 

 the liquid filled with bubbles and the air in the whole apparatus 

 in circulation. From time to time the color of the solution is 



