370 
McLean . — Field Studies of the Carbon 
to have the same volume of air pass through the two sets of apparatus 
during each test period, the following procedure was adopted. 
At the end of the first half-hour the bubble tubes in the Pettenkofer 
tubes of the control and the test apparatus were interchanged, this operation 
necessitating the stopping of the flow of air for less than one minute. 
At the end of the first hour, the glass stop-cocks connecting the test and 
control apparatus with the aspirator were interchanged so that the valve 
which had regulated the flow through the test apparatus during the first 
hour would now regulate the flow through the control, and vice versa. At 
the same time that this was being done, the suction tube drawing air from 
both sets of apparatus was transferred to a full aspirator tank, so that the 
suction on the apparatus was the same at the beginning of the second hour 
as at the beginning of the first. These operations necessitated the stopping 
of the flow of air for nearly three minutes. At the end of the third half- 
hour the bubble tubes of the Pettenkofers were again interchanged, thus being 
returned to their original positions as at the beginning of the test. By the 
above procedure the amount of air flowing through the two sets of 
apparatus was made measurably the same. 
During each hour the rate of flow of air gradually diminished from 
twenty to eighteen litres per hour, due to loss of head in the aspirator tank. 
This hourly cycle in the rate of flow was of minor importance, since the rate 
was at all times sufficiently rapid to amply supply with air the leaves under 
test. This change in rate was moreover the same for both test and control. 
At the end of each two-hour period, the Pettenkofer tubes and barium 
hydroxide wash bottles were renewed. The glass stop-cocks were returned 
to their original positions, the suction tube was attached to a full aspirator 
tank, and the bubble tubes used in the Pettenkofer tubes were cleaned, 
as stated above, before a new test was begun. All of these manipulations 
took, on an average, about five minutes. 
Titration Apparatus. 
After removal from the apparatus the test and control Pettenkofer 
tubes and the barium hydroxide wash bottles were decanted into flasks 
which were closed and allowed to stand until the precipitated barium 
carbonate had settled out. Then the content of each flask was tested. 
Samples for testing were siphoned into a calibrated pipette, care being 
taken not to disturb the precipitated carbonate. The arrangement of this 
apparatus was similar to that employed by Copeland 1 to determine the 
concentration of barium hydroxide solution. It was so arranged that all of 
the air inside the apparatus was freed from carbon dioxide. The solution 
to be tested was first siphoned into a calibrated pipette, the capacity 
1 Copeland, E. B. : Chemical Stimulation and the Evolution of Carbon Dioxide. Bot. Gaz., 
xxxv. 81-98, 160-83, I 9°3* 
