60 
The errors in usino- pipettes may be especially manifest in measuring • 
solutions of higher densit}^ or greater viscosity than water. In fact it 
is quite impossible to obtain even approximately correct results in 
measuring such solutions as ghu*erin, blood serum, etc., from delivery 
pipettes. This fact led Ehrlich to use the capacity pipettes of von 
Ley bold in measuring these substances in making his unit for diphtheria 
antitoxin. 
I have found the methods which I shall now describe verv useful in 
the accurate work necessarv in making a Government standard* and 
believe their simplicity and accuracy will commend them to others. 
Met}i()d of 'mincj capacity pipettes . — The pipettes are fitted with a 
rubber bulb at their upper end and held in an ordinary retort stand, 
as ma}" be seen bv reference to the illustration (tig. 12). 
The rubber bidb is clamped between the jaws of the compressor. 
The fluid is drawn up into or expelled from the pipette by tightening 
or loosening the screw. By this means we have an al^solute control 
over the height of the column of fluid in the pipette. The rubber 
bulb takes the place of the suction usualL applied by the mouth and 
is used both to till and to empty the pipette. 
In using, the free end of the pipette is immersed into the fluid to be 
measured, and by loosening the screw the column of fluid is gradually 
brought approximateh" to the mark. The operation is repeated once 
or twice so as to wash the pipette with the fluid to be measured, which 
is flnally drawn a little above the graduation mark. Care must be 
taken not to wet the interior of the pipette at ain^ time during the 
process of measuring more than 1 to 2 mm. above this mark, because 
the extra wetted surface would furnish an appreciable error. By 
holding the pipette in the retort stand and using the rubber bulb as 
shown, such accidents are much less likely to happen than is the case 
AV'hen the mouth is used to suck the fluid up and down. 
The outside of the pipette is now carefullv wiped with a few thick- 
nesses of sterile gauze. Then by placing the gauze to the free end of 
the pipette the column of liquid may l)e slowly drawn down until the 
meniscus rests directly upon the line. 
The contents of the pipette are now emptied into the diluting fluid, 
which has previously been measured out, and the pipette must be 
washed out thoroughly by drawing the fluid up and down several 
times, again using the rubber bulb for this purpose. 
The flask containino- the dilution is now agitated so as to obtain a 
uniform mixture of solution, and the pipette is again washed in and 
out so that the fluid which adheres to and remains in the pipette is the 
same dilution as the mixture in the flask. A larger quantity of dilu- 
tion is always made than required, so that alicpiot parts will represent 
ver}" definite quantities of the original substances. 
