ON THE NATURE OP FERMENTATION. 186 



bacteria. The circumstance that they arc minute must not make us 

 shut our eyes to this truth. You sometimes hear bacteria spoken 

 of as if they were all alike. The fact that some do not move and 

 others do, is one indication of a difference between them. Another 

 indication of a difference is, that some bacteria will thrive in a 

 medium in which others cannot live. For instance, the Bac- 

 terium lact'is refuses to live at all, according to the more careful 

 experiments I have been lately making, in Pasteur's solution; 

 the very fluid provided by Pasteur for bacteria, torulse, and other 

 fungi to live in, is a medium in which the Bacterium lactis re- 

 fuses to grow at all, although many bacteria grow in it with 

 rapidity. That is clear evidence that this is a different kind of 

 bacterium from those which both thrive and move in Pasteur's 

 solution. You will observe, also, it is somewhat peculiar in the 

 form of the segments ; they are oval, and not so rod -shaped as 

 bacteria generally. These you will always find in milk when it 

 is souring. 



But, gentlemen, neither the souring of milk nor the organism 

 which is found associated with that change is the result of any 

 inherent tendency in the fluid. This is a flask of boiled milk 

 prepared on August 27th, with the same arrangements for 

 ensuring purity of the vessel and excluding dust that we 

 had in the flask of Pasteur^s solution. It has not coagu- 

 lated ; it has undergone none of the changes to which I 

 have alluded. There has been no butyric fermentation, no 

 Oidiiim lactis has formed upon it, no putrefaction has oc- 

 curred. This milk is as sweet as when it was first prepared; 

 and if you were to examine it with the microscope, you would 

 find in it no organism of any kind. Prom this same flask, with 

 precautions with which I will not detain you, I have charged vari- 

 ous 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 fermentation. As 

 regards boiled milk, this is sufficient evidence that the lactic fer- 

 mentation is not something to which the liquid is spoiitaneously 

 prone; it requires something to be introduced into it from with- 

 out. Por you must not suppose that the boiling has rendered 

 the milk incapable of souring. All that it requires is the intro- 

 duction 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 proneuess of boiled milk 

 to souring and also the genuine fermentative character of that 

 change as indicated by the faculty of self- multiplication of the 



