ON THE NATURE OF FERMENTATION 349 
lactis that the dilution had been made, and not with reference to these other 
organisms relatively so rare. It happened that we saw in the souring milk 
before making the dilution that there was another kind of bacterium present, 
a moving kind different from the Bacterium lactis ; it was in every field, but 
not nearly so numerous as the Bacterrum lactis, and, consequently, it did not 
occur in the one milk that curdled. 
Now, therefore, we had every reason to hope that we had got the ferment 
pure, and thus we had the opportunity of performing other experiments ; and 
the last experiment that I shall mention is this. Having induced the lactic 
fermentation in another glass of pure boiled milk by means of our presumably 
pure ferment, and estimated the number of bacteria per minim, I diluted with 
boiled water accordingly and then proceeded as follows :—These five covered 
test-tubes which you see before you, containing boiled milk in their lower part, 
were inoculated each with a drop calculated to contain two bacteria ; these other 
five similar test-tubes were inoculated each with a drop calculated to contain 
one bacterium ; these five liqueur-glasses were also inoculated with drops each 
calculated to contain one bacterium ; and one other liqueur-glass with a drop 
calculated to contain four bacteria. The result was that the specimen with 
the drop calculated to contain four bacteria soured and curdled in a few days ; 
and all these five calculated to have two bacteria to a drop curdled also in a few 
days. The milk, you see, is perfectly solid. You will also observe that no 
change has taken place except the lactic fermentation, no Ozdium lactis has 
grown, and no other alteration has taken place ; it is as pure in whiteness as 
when it was first coagulated. I may here mention that, although all these 
glasses of milk coagulated, they did not do so at the same time. There was 
a time in the twenty-four hours during which the coagulation went on, in which 
I hoped that some of them were going to be permanently fluid, implying, as 
you would expect, that the particles of the ferment were not uniformly dis- 
tributed ; some had more than others, though each happened to have at least 
one. But, of the five test-tubes calculated to have only one bacterium on the 
average to each inoculating drop, three have remained fluid, and so have two 
of the liqueur-glasses ; so that, of the ten calculated to have on the average 
one bacterium each, exactly five, it so happens, have remained fluid without any 
curdling. I may consider myself somewhat fortunate that I have succeeded 
in bringing these specimens all the way from Edinburgh in this condition. 
I will now deprive this one of the protection in which it has hitherto lived. 
[Mr. Lister, having removed the glass shade and glass cap from one of the liqueur 
glasses, proceeded to drink part of its contained milk.} It is perfectly sweet. 
It has a slight flavour of suet, which M. Pasteur has described as resulting from 
