PASTEUR, ON FERMENTS. 257 



spheric dust, as well as that of the Infusoria or of the Muce- 

 dinea. 



Milk exhibits still more interesting properties. I have 

 said that, before filling the flask with air which has been sub- 

 jected to a red heat, and hermetically closing it, I caused the 

 urine to boil for two or three minutes. This duration of the 

 ebullition is sufficient, and everything leads me to believe 

 that even less careful precautions will suffice to deprive of all 

 viability the germs which may have fallen into the urine 

 subsequent to its emission. 



This being granted, let us repeat, without any change, 

 the operation above described — now, however, not upon 

 urine, but upon fresh milk ; that is to say, after this fluid 

 has been boiled for two or three minutes, and the flask has 

 been refilled with air heated to redness, let us keep it closed 

 at a temperature of 30°. 



After a variable lapse of time — generally of three to ten 

 days — the milk in all the flasks thus prepared will be found 

 coagulated. Under the prevalent views respecting the phe- 

 nomenon of the coagulation of milk, there is nothing in this 

 circumstance to excite surprise. When milk, it is said, is 

 exposed to contact with the oxygen of the air, the albu- 

 minous element is altered and acts as a ferment. This fer- 

 ment reacts upon the sugar of the milk, and transforms it 

 into lactic acid, which then precipitates the casein. This is 

 the cause of the coagulation. In reality, however, things 

 are quite otherwise. For if one of these flasks in which the 

 milk is coagulated be opened, it is obvious, on the one hand, 

 that the milk is as alkaline as fresh milk ; and on the other 

 — a circumstance tending to encourage the belief in sponta- 

 neous generation — that the milk is filled with Infusoria, 

 most frequently with Vibrios, as much as ^ th millimeter in 

 length. As yet I have not met with any vegetable produc- 

 tion under these circumstances. 



From these facts we must admit — First, that the pheno- 

 menon of the coagulation of milk, as I hope shortly to demon- 

 strate more clearly, is a phenomenon upon which we have 

 had but very imperfect notions. Second, that Vibrios may 

 arise in a liquid of the nature of milk which has undergone 

 ebullition for several minutes at a temperature of 100° C, 

 although this is not the case with respect to urine, nor to a 

 mixture of sugar, water, and albumen. Is it the case, then, 

 that under particular conditions we may have spontaneous 

 generation? We shall soon see how far this conclusion 

 would be erroneous. Let the milk be boiled, not for two, but 

 for three, four, or five minutes, and it will be found that the 



