376 ON THE LACTIC FERMENTATION 
by all to afford a conclusive demonstration that the particular species of bacterium 
which we have been studying is really the cause of this special fermentation.’ 
It is true that this proof applies only to one particular kind of fermentative 
change. But the same method will, I believe, be found applicable in other 
instances for the purpose of ascertaining how far the rule is universal that 
a true fermentation, by which I mean one characterized by the faculty of self- 
multiplication of the ferment, is caused by the development of an organism. 
In the meantime, as a contribution of a definite character, so far as it goes, to 
the elucidation of the nature of fermentative changes, so fraught with interest 
at the present day alike to the physician and the surgeon, I trust it will be not 
considered unwelcome to the Pathological Society. 
There are some other points which came out in the course of this inquiry 
to which I should like to allude. One of these has reference to the cause of 
the odour of souring milk. In the lactic fermentation the sugar of milk is 
resolved into lactic acid by a mere rearrangement of its constituent atoms, 
one atom of sugar of milk going to form four atoms of lactic acid, without gain 
or loss. Now the chemist tells us that lactic acid is non-volatile, and accord- 
ingly this acid in the pure condition is absolutely odourless. Why, then, should 
souring milk have any odour? In fact this investigation has proved, as a result 
of isolating the true lactic ferment, that the smell of souring milk is principally 
due to the products of some other concomitant fermentation or fermentations. 
For when a glass of pure milk is made to sour under the influence of the Bacterium 
lactis pure and unmixed, the odour that results is extremely trifling. Neverthe- 
less the air in the glass shade over a vessel containing milk in such circumstances 
has a shght sharp odour ; and it seemed worth while to ferment a considerable 
quantity of milk in this way and subject it to distillation, so as to ascertain, 
if possible, the nature of the odorous ingredient. Having curdled boiled milk 
in a purified flask by means of pure Bacterium lactis, I mixed six ounces of the 
recently formed clot with five ounces of distilled water, and having introduced 
it into a retort, heated it for some hours in a bath of boiling water, the distillate 
being collected in a receiver kept cool with cloths over which cold water con- 
* Strictly speaking, the expression should be, not ‘ the cause’, but ‘ the cause under ordinary cir- 
cumstances’. For our facts do not of course exclude the possibility of the existence of some other 
ferment which might produce the same effect upon milk if the circumstances were favourable to its 
development, as, for example, by the exclusion of the Bacterium lactis. In truth, I once obtained as 
the result of taking milk directly from the cow into a purified vessel, a minute moving bacterium, which, 
though it developed very slowly in comparison with the Bacterium lactis, produced in time a sour curd, 
which I have little doubt was due to the change of milk-sugar into lactic acid. But thus interpreted, 
the terms in the text may be considered admissible ; just as we speak of the yeast plant as the cause 
of the alcoholic fermentation, although we know that the Mucor vacemosus, when growing unmixed in 
a saccharine solution, gives rise to the same fermentative change. 
