GENERAL APPARATUS AND MANIPULATION 179 



is suspended in such a way that it is held against the motor housing 

 by means of the rubber band. The tubes are turned once for about 

 each five revohitions. 



Agitation of a liquid in a capillary tube was effected by Bessey 

 et al. (]946) by touching the side of the tube to a rapidly rotating 

 nail head (page 250). 



For stirring small volumes of liquid in an open shallow vessel, 

 Kirk ( 1933) employed the tip of a glass needle drawn from the end 

 of a tube in which a piece of iron was sealed. The core of an electric 

 buzzer placed in proximity to the iron made the glass needle vibrate 

 and stirring was thus produced. This type of stirrer is manufactured 

 by Micro chemical Specialties Co. 



A particularly effective and convenient stirring device for small 

 volumes in open or closed vessels is the electromagnetic "flea" of 

 Linderstr0m-Lang and Holter ( 1931) . The "flea" consists of a sealed 

 glass spherical shell, about 1-2 mm. in diameter, filled with ferrum 

 reductum; stirring is effected by an electromagnet repeatedly turned 

 off and on by means of an interrupter. The core of the magnet is 

 placed near the outer wall of the vessel, and the lifting and dropping 

 of the "flea" provides the agitation. The arrangement employed in 

 titration is shown in Figure 64. The interrupter is not shown; it is a 

 small glass-enclosed mercury switch mounted on a pivot which is 

 connected to a movable strip of iron in the field of the magnet. When 

 the current is turned on, the magnetized core tips the iron strip, 

 which tilts the mercury switch and thus breaks the current. The iron 

 strip falls back when the core is no longer magnetized, and in so 

 doing it brings the mercury switch back to its original position, 

 which again completes the circuit, magnetizes the core, and starts the 

 process over again. "Fleas" are made by blowing a small bulb in the 

 end of a drawn-out piece of glass tubing, tapping a little ferrum 

 reductum down into the bulb, and sealing off the neck with a micro- 

 flame. The "fleas" may be cleaned by rinsing with water and cover- 

 ing them with fuming nitric acid. After washing well with distilled 

 water, the "fleas" are allowed to dry on a piece of filter paper. The 

 development of a brown stain of iron oxide on the filter paper under 

 a "flea" indicates that it has an incomplete seal and it should be 

 discarded. A method of testing the "fleas" suggested by Linderstr0m- 

 Lang and Holter ( 1940) is to place a drop of neutral bromothymol 

 blue solution on each one on a glass plate. Those not properly sealed 



