SMALLPOX AND COWPOX. 657 
keep at least three months. From time to time a 
single lot of virus will fail by the end of one month. 
Sometimes this is due to the glycerin, as when it has 
some chemical impurity, or simply that the glycerin is. 
not diluted sufficiently with water. We find one part 
of water to two of glycerin makes a good dilution. 
Bacteria in Vaccine. It is impossible to prepare vac- 
cine so that it is at the time of its removal free from 
bacteria. In fact, there are usually very large numbers 
of one or more varieties of bacteria present. When the 
stable and animals have been kept clean the bacteria 
comprise usually very few varieties; when dirty condi- 
tions prevail the bacterial varieties are more numerous. 
The number of bacteria found varies enormously. The 
largest number found by us was 126,360 in one loopful 
of vaccine virus, and the smallest number 528. Discrete 
vesicles at the borders contain many less bacteria than 
the confluent ones caused by the inoculation at the 
scarification. The pulp has many more bacteria than 
the contents of the vesicles. The period which elapses 
before glycerinated virus becomes sterile is also quite 
variable, but does not depend in any direct way upon 
the number of bacteria originally present. A very 
large number may disappear rapidly, and a few persist 
for a long time. 
After two or three weeks the number of living bac- 
teria is usually greatly diminished, but seldom totally 
destroyed. If we wait until the vaccine is surely sterile 
it is very apt to be also useless—that is, the vaccine 
bodies are dead also. 
In a very large experience we have learned that the 
number of bacteria present has little to do with the 
resulting vaccination. The character of the vesicles in 
42 
