186 PROFESSOR LISTER. 



ferment, xlnd on microscopic examination you would be sure to 

 find the Baciermm lactis present throughout the mass. 



But though the ferment which occasions the souring of milk 

 is present in tlie milk obtained from any dairy, it appears to be 

 by no means common in the world in 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 apartments 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 Bacterium 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 fermen- 

 tation is a rare ferment. So far from boiled milk being spon- 

 taneously prone to the change, it requires something to be intro- 

 duced 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 un- 

 boiled? ''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, according 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 spontaneous generation, and prone to produce these 

 and other fermentations, but which, by the act of boiling, we 

 deprive of this tendency. Therefore, with a view to meeting this 

 objection, the first part of my investigation was devoted to en- 

 deavouring to see whether or not milk, as it comes from the cow, 

 really does or does not contain materials tending to the develop- 

 ment of organisms or to fermentation of any kind. 



Aq exceedingly simple experiment will probably serve to con- 

 vince you to a considerable extent with regard to this matter. If 

 you go to a dairy where there is also a cow-house, take a couple 

 of clean bottles, and fill one with milk from the dairy and the 

 other with milk direct from the cow in the cow-iiousc, the milk 

 obtained from the dairy will be certain to sour, but the milk that 

 you get direct from the cow will very probably never sour at all. 

 It will probably acquire a nasty bitter taste, and will not have 



