350 ON THE NATURE OF FERMENTATION 
the oxidation of the oleaginous material of the milk. If any gentleman likes to 
taste it after the lecture, he can do so. 
Let me note this curious circumstance, that, of those specimens which did 
coagulate, those in the tubes coagulated considerably earlier than those in the 
more open vessels. At first, it seemed as if, for some strange reason, those in 
the open vessels were going to remain permanently fluid—even that which had, 
according to the calculation, four bacteria to the drop. I presume this is to 
be explained on the same principle as Pasteur has explained a corresponding 
fact with regard to the yeast plant. He has shown that, if a saccharine solution 
be put in a very thin layer in an open vessel with yeast, the yeast plant develops 
very rapidly, but very little fermentation occurs ; on the contrary, if it be put 
into a deep vessel, the development of the yeast plant does not go on so rapidly, 
but more fermentation results. He explains the fact in this way: that the 
yeast plant requires oxygen for its nutrition ; if it gets it easily, as it does in 
a shallow vessel in the air, it produces comparatively little effect in breaking 
up the sugar into its constituents, and vice versa. So here, in the test-tubes the 
carbonic acid accumulated, supposing any to exist, asin a well, and the Bacterium 
lactis had but little opportunity for getting oxygen. Accordingly here, 
just as in M. Pasteur’s experiments with a sugary solution with yeast 
in a deep vessel, the Bacterium lactis produced more rapidly its fermentative 
effect. 
But this, you say, is assuming that Bacterium lactis is the ferment. Now 
we are coming to that point. But first I have to mention an additional fact. 
For the satisfaction of others rather than for my own, I went through the 
laborious process of investigating portions of the contents of all these vessels ;. 
and I found that, in every one in which the lactic-acid fermentation had taken 
place, where there was curdling and souring, the Bacterium lactis was present ; 
and in no instance in which there was no lactic fermentation was any bacterium 
of any sort to be discovered. I believe that fact demonstrates that the Bacterium 
lactis is the cause of this very special lactic fermentation. Let us assume for 
a moment that there did exist some other material besides the Bacterium lactis 
in the milk capable of causing the fermentation ; that the lactic ferment was not 
the bacterium at all, but some chemical ferment. First of all, you will please 
to observe that we have from this experiment absolute evidence that the ferment, 
of whatever nature, is not in solution, but in the form of suspended insoluble 
particles. If the ferment had been in solution, every equal-sized drop of the 
water of inoculation would have produced the same effect. The fact that some 
drops were destitute of the ferment proves that that ferment was not in a state 
of solution. That is absolutely demonstrated. Now, suppose we admit, for 
