59 
A study of the histories of the g'uiuea pigs showing the presence of 
bacteria in the blood disclosed the fact that most of them were prob 
ably 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 the}" 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 
be obtained from the firm making them, it will be necessary to restand- 
arize them to determine several points. 
With delivery pipettes it is essential to know whether they are to 
be used by 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 by 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 by 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 by 1 kilo- 
gram of pure water at 1° C., the weighing being reduced to a vacuum. 
The volume of the true liter may be assumed equal to 1,000 c. c. 
Volumetric apparatus is for convenience usually graduated to con- 
tain or to deliver true cubic centimeters at higher temperatures than 
1° C., namely, at 15°, 17.5°, or 20° C., the standard temperature 
being marked upon the apparatus. The difi'erences between apparatus 
graduated on the true and on the Mohr svstems are as follow"s:^ 
If standard at 15°, Mohr apparatus is larger by 2.30 c. c. per liter. 
If standard at 17.5°, Mohr apparatus is larger by 2.36 c. c. per liter. 
If standard at 20°, Mohr apparatus is larger by 2.42 c. c. per liter. 
In ordinary rough-and-ready work all these refinements may be 
disregarded, but in certain determinations requiring great precision a 
neglect of these details may cause serious error. 
These matters were brought forcibly to my 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. 
