416 DILATOMETRIC METHODS 



Method of Adding Drops. The procedure for adding the drops 

 is as follows: 



1. Dip the tip of the pipette below the kerosene layer into the 

 solution. 



2. Rinse the pipette by drawing up and blowing out the solution 

 several times. » 



3. Draw the solution to the mark on the pipette. 



4. Raise the pipette until the tip is in the kerosene. 



5. Draw up about 0.1 /xl. kerosene into the pipette. 



6. Remove the pipette and wipe the outside with the edge of a 

 piece of filter paper. 



7. Blow out about half the kerosene while the tip is touching a 

 piece of filter paper. 



8. Dip the pipette into the gradient tube so that the tip is about 

 2 mm. below the surface of the medium. 



9. Blow out the pipette charge. 



10. Remove the drop from the tip by lifting the latter out of the 

 medium. The drop falls and reaches its equilibrium position usually 

 within 15 min. 



Method of Removing Drops. Drops may be removed in the 

 gradient tube by inserting a long thin glass rod with a fine point to 

 which the drops will adhere. They may then be carried up and 

 placed on the wall of the tube near the surface of the medium. It is 

 impossible to remove them entirely from the medium in this fashion 

 since the surface forces act to detach the drops from the glass tip 

 when this is attempted. The glass rod should be handled carefully 

 to avoid undue disturbance of the density gradient. 



Dr. Oliver H. Lowry instituted the practice of adding a little sand 

 (60-80 mesh) from a salt shaker to remove the droplets. This is an 

 effective method which disturbs the gradient less than the glass rod 

 procedure. 



Measurement of Drop Position. The position of drops is 

 determined by a horizontal microscope of long focal length mounted 

 on a micrometer stand (Fig. 151). The microscope (A) is pivoted 

 at jB; C is a calibrated rough adjustment rack with a range of 100- 

 150 mm., and £' is a clamp to hold the rack at any given setting. 

 The micrometer screw D enables fine adjustment, it has a vertical 



