16 



following reasons : — First, it is motionless, whereas the ordinary 

 putrefactive Bacterium — the Bacterium termo as it is called — passes 

 a considerable portion of its lil'e in active movement. Secondly, 

 it differs from it somewhat in form, the segments being more 

 oval and less rod-like. Thirdly, it refuses to live in liquids in 

 which ordinary putrefactive Bacteria thrive perfectly well. 

 Fourthly, if you boil a number of flasks of milk so as to sterilize 

 them, and subsequently introduce a drop of ordinary water, or 

 expose them to the air, the milk will still undergo decomposition, 

 but the lactic fermentation d>>es not take place, nor is the special 

 Bacterium found. He has given it the name of Baclermm Lactis, 

 and believes it to be found in dairies, and usually nowhere else. 



This is really a very important experiment, as of course so 

 far as it goes it supports the view that there are special forms of 

 Bacteria, associated with special kinds of fermentation, and pro- 

 bably with these only ; but is the Bacterium really the cause of 

 the fermentation ? 



Lister conceived the idea that if he could by any means 

 count the number of Bacteria in any given sample of souring 

 milk, he would by suitable dilution, be able to prepare a liquid 

 of such a strength that each drop would on an average contain 

 one Bacterium. Then he says, as it is practically certain that 

 these Bacteria would not be uniformly distributed through the 

 liquid; the result would be that some of these drops would con- 

 tain Bacteira and some would not. I am now speaking of course 

 of his Bacterium Lactis, the special Bacterium of the souring milk. 

 He succeeded in counting these Bacteria by a method similar to 

 that now adopted for counting the blood corpuscles, and he found 

 that it was necessary to add to the milk no less than a million 

 parts of boiled water to get the liquid of the dilution before 

 mentioned. Of this he added one drop to each of five glasses of 

 boiled milk. After a short time he found that one only out of the 

 five had soured, and in this one only were Bacteria discovered. 

 He repeated his experiments in various ways, but the result was 

 always the same. AVith'fermention Bacteria; without fermenta- 

 tion no Bacteria. Then he says, if the Bacterium is not the fer- 

 ment, what is it? Is it a soUd or a liquid? If it were a liquid 

 it Avould be uniformly diffused through the inoculating fluid, and 

 therefore fermentation would occur in all the glasses or in none. 



