MICROLITER DIVER TECHNIQUE 361 



Devices for Holding Pipettes. In order to allow convenient 

 adjustment of pipettes, clamps fitted to universal ball joints were 

 used by Holter (Fig. 125). The clamps were mounted on a horizontal 

 bar, under which a ground-glass plate was fixed and illuminated 

 from behind to furnish favorable light for the pipetting. Pipettes 

 which must be removed from their clamps to obtain the sample, or 

 for any other reason, should be marked so that they can be re- 

 clamped at exactly the same place in order to avoid the necessity of 

 recentering them with respect to the diver neck when the diver is 

 held in its clamp stand. When it is desirable to use several pipettes 

 in quick succession with the same diver, the pipettes must all be 

 aligned to the diver, and the diver stand must rest on a surface 

 smooth enough to enable it to be pushed under the various pipettes 

 without losing the alignment. A level glass plate on the top of the 

 table is recommended. 



Calibration of Pipettes. Pipettes of types 1 and 3 are calibrated 

 by iodometric (1 N potassium iodate) or acidimetric (1 N potas- 

 sium hydroxide) microtitration of a pipetted vol. of liquid using 

 0.01-0.02 A^ standard solutions for the titration. Type 2 pipettes 

 are calibrated by weighing paraffin oil delivered from the pipette 

 directly on to the pan of a microbalance. 



Calibration markings on types 1, 2, and 4 pipettes are placed far 

 up on the pipette in the form of strips of millimeter paper. If the 

 pipettes are made from a broken thermometer, the graduations on 

 the glass will serve. A thin-walled and expanded portion of the 

 pipette, 5-10 mm. long, should lie between the fine stem and the 

 graduated part (Fig. 125). Type 3 pipettes require only one mark, 

 since they are used for complete discharge. No very satisfactory 

 means has been found for placing the mark when it must be so 

 near the end as to enter the diver neck during pipetting. Loops of 

 hair, held by a tiny bit of picein or glass cement, are usually too 

 bulky. Glass ink covered with lacquer is soon worn off. The Carls- 

 berg Laboratory group prefer no markings at all, but, after the 

 pipette has been filled according to judgment under a binocular 

 microscope, the length of the liquid column in the pipette is meas- 

 ured to 0.1 mm. by means of a micrometer scale. The pipette is 

 emptied into the bottom of the diver, and if the volume was mis- 

 judged, the difference is made up by correspondingly changing the 

 volume of one of the neck seals or the mouth seal. When the pipette 



