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 523. 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 



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