METABOLIC PROCESSES IN THE PLANT. 



267 



mixture is frequently shaken, and after some time poured into the 



raised bottle (Fig. 98), which has a capacity of about 10 litres. 



The tube A-", containing fragments of potash, serves to prevent the 



clear baryta water from absorbing Carbon dioxide. The solution 



can be run oif into the burette 6, which carries at its upper end 



the potash tube A-', and the Pettenkofer's baryta tubes, after being 



well cleansed and thoroughly dried, are filled from the burette. 



The contents of these tubes are rapidly transferred at the end 



of each experiment to tall, well-closed cylinders, and after the 



precipitate has 



settled, we make 



two titrations 



of the clear 



su p e rnatant 



fluid, removing 



25 c.c. with a 



pipette for each 



determination. 



We use for the 



titrations (as 



also, of course, 



in titrating the 



original baryta 



water) a solution of Oxalic acid, con- 



taining 2'8636 gr. of the crystallised 



acid per litre, and of which 1 c.c. cor- 



responds with 1 mgr. of Carbon dioxide. 



The Oxalic acid solution is admitted to 



the baryta water from a burette pro- 



vided with a float. For the indicator, 



a few drops of phenolphthalein solution 



(100 c.c. of alcohol and 0'5 gr. of 



phenolphthalein) are always employed. 



The method enables us to determine the 



Carbon dioxide accurately to T \j mgr. 



The above method of investigation has 



frequently been tested in various direc- 



tions by myself and my students, and it has yielded very satis- 



factory results. 2 It remains, however, to be noted that the 



results of the observations made in the manner described are 



always somewhat too high. If, viz., we experiment without any 



FlG . 98 ._ Titrating apparatus . 



