DONALD'S DROP MEASUREMENT 59 



arrested : it is then cut above and flush with the steel 

 plate with a glass-cutting knife. The end of the pipette 

 will now have an external diameter corresponding with 

 that of the hole of the wire gauge. A wire gauge is a steel 

 plate pierced with standard holes which are numbered. 

 Various wire-gauges are on the market, e.g. the Stubbs 

 Lancashire, the Morse, and the Birmingham. The table 

 (see p. 58) from Donald's papers gives some of the data 

 for the Stubbs and Morse gauges (temp. = about 20 C.) : 

 The dropping should be done with the pipette held nearly 

 vertical and the drops should fall at a rate of about 

 one per second. The point of the pipette should be 

 untouched with the finger : it must be absolutely clean 

 and free from grease. For different liquids Donald has 

 worked out the following factors : 



Water . . .1-0 Peptone broth . .1-2 



Saline . . .1-0 Standard agglutinable 



Serum, human . 1-1 typhoid culture . .1-0 



Serum, guinea-pig . 1-06 Alcoholic antigens . . 2-5 

 Cerebro -spinal fluid . 1-02 



This means, for example, that 6 drops of peptone 

 broth measure the same volume as 5 drops of water or 

 saline, from the same diameter pipette. For steady 

 dropping the pipette should be furnished with a rubber 

 teat and may be " throttled " as described below. For 

 the measurement of any number of equal volumes, 

 Wright's method may be employed. A similar pipette 

 is used as in the drop method (Fig. 36, a, p. 260). Clean 

 mercury (see Appendix) is sucked into the pipette so as 

 to fill any convenient volume. A mark is then made on 

 the pipette with grease pencil at the upper limit of the 

 mercury column ; this forms the " unit volume." By 

 carefully tilting the pipette and regulating the flow with 

 the finger placed on the point, the mercury is brought so 

 that the lower end of the column corresponds with the 



