59 
A study of the histories of the guinea pigs showing the presence of 
bacteria in the blood disclosed the fact that most of them were prob- 
abh^ either terminal or post-mortem infections. 
METHOD OF USING THE PIPETTES. 
In order to obtain accurate results with pipettes it is important to 
use them in the same wa}" that they have been calibrated. This applies 
both to pipettes which have been graduated “to deliver” a given 
volume as well as to pipettes graduated “to contain” a given volume. 
If the method by which the pipettes were graduated can not readily 
l)e obtained from the tirm making them, it will be necessar}^ to restand- 
arize them to determine several points. 
With deliveiy pipettes it is essential to know whether they are to 
be used ly blowing out the last drop or by touching the point of the 
pipette to the surface in order to remove the last drop that adheres to 
the tip. 
The time allowed for drainage, after the pipette is emptied, varies 
with the rate of flow and also with the standard methods used b}" dif- 
ferent countries. 
It is furthermore very important in accurate work to know whether 
pipettes have been graduated in accordance with the Mohr liter or the 
true (French) liter. 
The Mohr liter is the volume occupied b}" 1 kilogram of pure water 
at 17.5° C., weighed in air against brass weights, the air being under 
normal pressure and at 17.5° C. The volume of such a liter is 
1,002.36 c. c. 
The true or so-called French liter is the volume occupied b}" 1 kilo- 
gram of pure water at 4° C., the weighing being reduced to a vacuum. 
The Amlume of the true liter may be assumed equal to 1,000 c. c. 
Volumetric apparatus is for convenience usualh" graduated to con- 
tain or to deliver true cubic centimeters at higher temperatures than 
4° C., namehy at 15°, 17.5°, or 20° C., the standard temperature 
being marked upon the apparatus. The difl'erences between apparatus 
graduated on the true and on the Mohr s}Tstems are as follows:" 
It standard at 15°, Mohr apparatus is larger by 2.30 c. c. per liter. 
If standard at 17.5°, Mohr apparatus is larger b\^ 2.36 c. c. per liter. 
If standard at 20°, Mohr apparatus is larger b}- 2.42 c. c. per liter. 
In ordinary rough-and-read}" work all these refinements ma}^ be 
disregarded, but in certain determinations requiring great precision a 
neglect of these details may cause serious error. 
These matters were brought forcilily to mv attention in the work 
of making the standard unit for diphtheria antitoxin. 
« I am indebted to Dr. L. A. Fischer, Chief of the Division of Weights and 
Measures, Bureau of Standards, for these figures. 
