342 ON THE NATURE OF FERMENTATION 
his first statement, that the caseine is the ferment, might lead you to suppose 
that he is inclined to the former view.' If this were the case, as there is caseine 
always in the milk, there should always be the lactic-acid fermentation. But 
it was pointed out long ago by M. Pasteur that, if you examine any specimens 
of souring milk with the microscope, you find little organisms.2 These, when 
you come to look at them carefully, you see to be obviously of the nature of 
bacteria. Bacteria may either have the faculty of motion or they may not. 
This particular bacterium is a motionless bacterium, so far as I know; still it 
has the essential nature of a bacterium: a microscopic fungus, multiplied by 
fissiparous generation, the lines of segmentation being transverse to the longitu- 
dinal axis of the organism. I have ventured to give to this little organism the 
name Bacterium lactis; for, gentlemen, no doubt there are different kinds of 
bacteria. The circumstance that they are minute must not make us shut our 
eyes to this truth. You sometimes hear bacteria spoken of as if they were all 
alike. The fact that some do not move and others do, is one indication of 
a difference between them. Another indication of a difference is, that some 
bacteria will thrive in a medium in which others cannot live. For instance, 
the Bacterium lactis refuses to live at all, according to the more careful experi- 
ments I have been lately making, in Pasteur’s solution; the very fluid pro- 
vided by Pasteur for bacteria, torulae, and other fungi to live in, is a medium in 
which the Bacterium lactis refuses to grow at all; although many bacteria grow 
in it with rapidity. That is clear evidence that this is a different kind of bac- 
terium from those which both thrive and move in Pasteur’s solution. You will 
observe, also, it is somewhat peculiar in the form of the segments ; they are 
oval, and not so rod-shaped as bacteria generally. These you will always find 
in milk when it is souring. 
But, gentlemen, neither the souring of milk nor the organism which is 
found associated with that change is the result of any inherent tendency in the 
fluid. This is a flask of boiled milk prepared on the 27th of August, with the 
same arrangements for ensuring purity of the vessel and excluding dust that we 
had in the flask of Pasteur’s solution. It has not coagulated ; it has undergone 
none of the changes to which I have alluded. There has been no butyric fer- 
mentation, no Ozdium lactis has formed upon it, no putrefaction has occurred. 
This milk is as sweet as when it was first prepared ; and if you were to examine 
it with the microscope, you would find in it no organism of any kind. From 
this same flask, with precautions with which I will not detain you, I have charged 
* Vide Miller’s Elements of Chemistry, third edition, vol. iii. 
* Vide ‘ Mémoire sur le Fermentation appelée Lactique’, Annales de Chimie et de Physique, 3™¢ 
série, tome lii, 1858. 
