36 PLANT RESPIRATION 



through each pipette in any desired direction (according to the 

 position of the bulb). The steel screw/, which is entirely anal- 

 ogous to the screw e. serves the pipette D for delicate adjust- 

 ments of the mercury column in the tube above. The pipette 

 D is an absorption pipette but the pipette E is an explosion 

 pipette. In the upper part of the bulb are fused platinum 

 wires, the ends of which are 1-2 mm. apart. Outside the bulb 

 the platinum wires are joined to the poles of a Rumkorf spark 

 coil. The latter is itself joined to a zinc-carbon element. The 

 element is filled with a chromic acid solution which is prepared 

 from 92 grams of potassium dichromate, 93.5 cc. of cone, sul- 

 phuric acid, and 900 cc. of water. For taking readings on the 

 measuring tube there is used a horizontal reading microscope 

 which is placed on set screws and moved by a slide and gearing 

 at a height of 40-50 cm.^ The exact fixation on the object is 

 done with a rack and pinion. The instrument is so focused that 

 each millimeter division of the measuring tube is divided into 

 20 parts by the ocular micrometer of the microscope. Small 

 analysis tubes, closed above and with fused supports (R), are 

 needed in addition to carry out the analyses. The length of 

 each tube is about 6-7 cm. (without supports). 



The measuring tube, the two gas pipettes D and E, and the 

 bath C are filled with pure, dry mercury by the use of the two 

 bulbs G and H. After filling the gas pipettes and the bath, 

 only 2-3 cc. of mercury should remain in the pear-shaped bulb 

 G. The glass cylinder B is filled with distilled water and 

 covered with a glass plate to protect the water from dust. From 

 time to time water which has become somewhat turbid is 

 replaced by fresh. The bulb of the pipette D is filled to a third 

 with about 30% potash [or NaOH]. To accomplish this an 

 analysis tube is prepared with 3-4 cc. of potash and the lye sealed 

 off with mercury. The analysis tube is carried to the mercury 

 bath C with the aid of an iron ladle [cf. manuals of gas analysis 

 methods) and placed under the mercury on the upper tube of 

 the absorption pipette so far that the end of the pipette dips 

 into the potash. The bulb G is now lowered, the stopcock n 



1 Model of Leitz. Since the side of the measuring tube that is divided into millimeters is 

 only about 20 cm. long, the adjustable cathetometer. which can be set to read lower, could 

 also be xtsed. 



