ON THE NATURE OF FERMENTATION 343 
various purified liqueur-glasses. This one has been charged for more than 
four weeks, yet the milk remains fluid, you observe, although there is abundantly 
free access of air to it. The oxygen of the air and the caseine which still exist 
in the boiled milk have together been unable to bring about the lactic fermenta- 
tion. As regards boiled milk, this is sufficient evidence that the lactic fer- 
mentation is not something to which the liquid is spontaneously prone ; it 
requires something to be introduced into it from without. For you must not 
suppose that the boiling has rendered the milk incapable of souring. All that 
it requires is the introduction of the appropriate ferment. If you were to touch 
the edge of the milk in this glass with the point of a needle dipped in souring 
milk from a dairy, within two or three days the whole would be a sour clot, 
showing both the proneness of boiled milk to souring and also the genuine 
fermentative character of that change as indicated by the faculty of self- 
multiplication of the ferment. And on microscopic examination you would 
be sure to find the Bacterium lactis present throughout the mass. 
But though the ferment which occasions the souring of milk is present in 
the milk obtained from any dairy, it appears to be by no means common in the 
worldin general. Suppose you take a series of glasses of boiled milk like these, 
and introduce into them a series of drops of ordinary unboiled water, you will 
get fermentation in them. If you put into each, for instance, a drop as large 
as a quarter of a minim, you will have a fermentation in every one, and an 
organism in every one; but you will neither have, according to my experience, 
the lactic-acid fermentation nor the Bacterium lactis. You will have bacteria 
of other sorts ; fermentations of other kinds. Again, suppose you take a series 
of such glasses, take off the glass shades and the glass caps, in different apart- 
ments or at different times, and expose the milk to the air-dust for half an hour ; 
you will get fungi and bacteria of various sorts, but, according to my experience, 
you will not get the Bacterrum lactis; nor will you get the lactic fermentation. 
And thus it turns out, so far as boiled milk is concerned at all events, that the 
ferment that brings about this particular fermentation is a rare ferment. So 
far from boiled milk being spontaneously prone to the change, it requires some- 
thing to be introduced from without which is a rarity both in ordinary water 
and in ordinary air. 
But then, it may be urged, indeed such arguments have been used, this 
may be very true for boiled milk, but how about unboiled ? ‘ May it not be 
that, by boiling the milk, you have destroyed certain chemical ferments, purely 
hypothetical we must admit, but which we think likely to exist ?’ For, accord- 
ing to the views of some persons, it may be that in the unboiled milk there 
may exist certain chemical substances prone to evolve into organisms by spon- 
