348 ON THE NATURE OF FERMENTATION 
manner. A circular covering glass, just half an inch in diameter, was used. 
Of course, we know how many square thousandths of an inch there are in the 
area of this little glass. We also know by the micrometer how many thousandths 
we have across the field of our microscope, and, therefore, by calculation we 
know how many square thousandths there are in our field, and thus we can 
tell how many fields there are in the covering glass. To measure the liquid, 
I used this little syringe, with the piston rod in the form of a screw, on which 
revolves a disc, graduated for rooths of a minim; by which means you can, 
with perfect precision, emit 1-1ooth of a minim, or 2—1ooths, or any number 
you choose. I found that 2—100ths, or 1-5o0th, exactly occupied the covering 
glass ; so that, when it was put down upon a glass plate, with 1-5oth 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 magnifying power ; so that prac- 
tically the liquid was all under the covering glass. I knew, therefore, that there 
was I-50th of a minim under the covering glass. If, then, I counted how many 
bacteria 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 1-50th of a minim ; 
and, consequently, [ 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 inocu- 
lated 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 
lactis in abundance; the others did not curdle, underwent no fermentation 
whatsoever, and had no bacteria in them. ‘You may say, perhaps, ‘ 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 many 
other kinds of organisms undoubtedly did exist in the milk, yet most of them 
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 Bacterium 
