ON THE NATURE OF TERMENTATION. 191 



2-lOOths, or l-50th, exactly occupied the covering glass; so that, 

 when it was put down upon a glass plate, with 1-5 0th of a minim 

 interposed, the rim of fluid round about the covering glass was not 

 one quarter of the diameter of the field, using the highest magnify- 

 ing power; so that practically the liquid was all under the covering 

 glass. I knew, therefore, that there was 1-5 0th of a minim 

 under the covering glass. If, then, I counted how many bac- 

 teria there were in a field, taking a number of different fields and 

 striking an average, I could ascertain how many bacteria there 

 were on the average in a field ; therefore, by calculation, how 

 many there were under the covering glass ; or, in other words, 

 how many there were in the l-50th of a minim; and, conse- 

 quently, I knew how much boiled water I ought to add in order 

 that the drop, of whatever size I might wish it to be, should 

 contain, on the average, one bacterium, and one only. This 

 being done with a particular specimen of souring milk, I found 

 that it was needful to add no less than one million parts of 

 boiled water to the milk to ensure that there should be rather 

 less than one bacterium, on the average, to every drop. Then 

 with drops of that size I inoculated five glasses of boiled milk, 

 and the result was that out of the five only one curdled ; but one 

 did curdle and soured, and that one had the Bacterium ladis in 

 abundance ; the others did not curdle, underwent no fermentation 

 whatsoever, and had no bacteria in them. You may say, per- 

 haps, " How was it that there were none of these numerous 

 different organisms and fermentations that you have been showing 

 us?" Simply for this reason, that although these existed, and 

 one of them existed probably in every two or three minims of 

 the milk, yet they were in exceedingly small proportion to the 

 Bacterium lactis, so that you might have searched, perhaps, for 

 a whole day, with the high power of the microscope which it 

 was necessary to use, and never discovered one. We are apt to 

 forget how difficult it is to find these minute objects with high 

 powers of the microscope, unless they are very numerous indeed. 

 Therefore, when we came to dilute the milk with a million parts 

 of water, the chances of getting anything but the Bacterium 

 lactis were exceedingly small. It was with reference to the Bac- 

 terium lactis .that the dilution had been made, and not with 

 reference to these other organisms so exceedingly small in quan- 

 tity. It so happened that we saw in the souring milk before 

 making that dilution that there was another kind of bacterium 

 present, a moving kind different from the Bacterium lactis ; it was 

 in every field, but not nearly so numerous as the Bacterium lactis 

 and, consequently, it did not occur in the one milk that curdled. 

 Now, therefore, we had every reason to hope that we had got 

 the ferment pure, and thus we had the opportunity of performing 

 other experiments ; and the last experiment that I shall mention 



