ON THE NAFURE OF FERMENTATION 341 
tion. Blood and water constitute a mixture highly putrescible, very much 
more so than blood itself. But in this flask we have had mixed with water the 
contents of one of the liqueur-glasses of unputrefied blood like that before shown 
to you. The water, however, had been previously boiled, so as to kill any 
organisms in it; boiled and cooled under the protection of a cotton cap, and 
then, the cotton cap being raised, careful provisions (into which I must not 
enter) against the entrance of dust being taken, the clot was spooned into the 
water ; a fresh cotton cap, perfectly pure, was put on, and so we got, I believe 
for the first time, a permanent cold watery extract of blood, and here it retains 
the same brilliant clearness that it had in the first instance, more than a month 
ago. Mere water, therefore, is as inadequate to induce the putrefactive fer- 
mentation of blood as are the gases of the air. 
But the fermentation which I have been especially investigating has not 
been the putrefactive, but one which seemed to me more convenient for the 
purpose, the lactic fermentation, by means of which milk sours and curdles, 
through conversion of the sugar of milk into lactic acid. This is a curious 
instance of a chemical transformation. The composition, as regards the pro- 
portions of the three elements, carbon, hydrogen, and oxygen, remains identically 
the same ; but those of you who are chemists understand what I mean when 
I say the atomic weight of the lactic acid is one fourth of the atomic weight of 
the sugar of milk. Each atom of milk-sugar is resolved into four simpler atoms 
of lactic acid. Now, it may be naturally supposed, if you observe what happens 
in a portion of milk obtained from a dairy, that there is an inherent tendency 
in the milk to this souring and curdling. If you get milk from a dairy and keep 
it long enough, it is certain to turn sour and curdle: then, after a while, there 
comes a certain mould upon the surface, the Ordium lactis, which constitutes 
the sort of bloom there is upon a cream cheese ; then comes on, often simul- 
taneously with the growth of this mould, the butyric fermentation, in which 
butyric acid is produced ; and afterwards, if you keep the milk long enough, 
it will probably putrefy. When you see, time after time, specimens of milk, 
taken from various dairies, undergo this succession of alterations, you may be 
tempted to suppose that these were changes to which the milk was disposed 
from its own inherent properties as it comes from the cow’s udder. The late 
eminent Professor of Chemistry in this College, Professor Miller, in his excellent 
work on Chemistry, states that the ferment of the lactic-acid fermentation is 
the caseine of the milk. I am bound to say, however, in justice to Professor 
Miller, that he also adds that M. Pasteur has expressed his belief that there 
exists an organic living ferment which produces this fermentation ; but Professor 
Miller does not profess to decide between these two opinions. On the contrary, 
