4 GENERAL METHODS 



liquid remains in the tip. For blow-out delivery this final drop is 

 expelled. The expulsion is conveniently effected by closing the 

 upper end of the pipette with the right forefinger and warming the 

 bulb by gripping it with the left palm. The expansion of air in 

 the bulb forces the last drop of water out of the tip. For all 

 pipettes below 5 c.c. blow-out delivery should be used. Unless 

 all of the pipettes in the laboratory are calibrated for either blow- 

 out or drainage delivery each pipette must be etched " Blow-out " 

 or " Drainage." 



USE OF PYCNOMETERS 1 



The volume held by a pycnometer is determined by weighing 

 the water which it contains and multiplying this weight in grams 

 by the volume of 1 gram of water at the observed temperature 

 (i.e., by 1.0028 if temperature is 20). The density of any liquid 

 is determined by weighing the pycnometer full of the liquid 

 and dividing by the volume. The density is thus determined in 

 absolute units, the density of water at 4 being taken as 1. 



Urine specific gravities usually refer to water at the same tem- 

 perature (rather than water at 4) as unity. In this case the weight 

 of urine held by the pycnometer is divided by the weight of water 

 held at the same temperature. Such a specific gravity, if taken at 

 25, for example, is indicated by Dff . If water at 4 is taken as 

 the unit, as is done in tables of densities of acids, alcohol, etc., the 

 density is indicated by D 2 ^. 



Any vessel into which a volume of liquid may be accurately 

 measured may serve as a pycnometer. A pipette calibrated to 

 contain 1, 2 or more cubic centimeters may be used 



USE OF THE BALANCE 2 



See that the balance is perfectly level, as indicated by the 

 plumb-bob or spirit-level; if it is not, make it so by turning the 

 adjusting screws at the right and left front corners. > 



Always place the object to be weighed on the left-hand pan7and 

 the weights on the right-hand pan. One reason for this is to 

 equalize errors through possible inequalities in the length of the 



1 Medical War Manual, No. 6, 1918, p. 73. 



2 Ibid., p. 68. 



