374 ON THE LACTIC FERMENTATION 
souring, the taste of a little portion of the curd removed for investigation was 
sour and its reaction sharply acid, and under the microscope, in every instance, 
bacteria with the characters of Bacterium lactis were found, but no other organ- 
ism. And in the case of the remaining five glasses, where the milk was still 
fluid and unchanged in appearance, the air in the glass shade had merely the 
slight odour of suet, which Pasteur long since pointed out as resulting from 
oxidation ; the taste was that of fresh milk, and the reaction showed the peculiar 
form of neutrality which fresh milk exhibits, both blue and red litmus paper 
acquiring an intermediate purple tint." And on subjecting the contents of 
each to microscopic examination, I could discover under a protracted search 
no organisms of any kind. I have brought before the Society one of these last 
glasses (still under the protection of its glass cap and shade) to show that even 
after the lapse of nearly four months the milk remains fluid and unaltered. 
I need hardly say that it required considerable care to bring these glasses from 
Edinburgh without having their contents spilled. Nevertheless this is one 
which was successfully transported ; and I have placed under one of the micro- 
scopes a drop of its milk, in which it will be seen that there are no organisms 
present, and that the only alteration discoverable is that some of the milk 
globules have assumed an angular form as the result of evaporation. 
Here, again, is one of the glasses which underwent the lactic fermentation, 
and its appearance is in truth as remarkable as that of the other. We know 
that if curdled milk is kept under ordinary circumstances it soon loses its original 
characters. The Ozdiwm lactis grows upon its surface, the Penicillium glaucum 
or some other common mould probably shows itself, and the curd acquires first 
a cheesy and then a putrid smell, accompanied with great alterations of its 
aspect. Here, however, the lactic fermentation having occurred single and 
alone, we have a pure white curd to this hour, as if the milk had curdled yester- 
day, and any little smell which there is in the air of the glass shade is a slight 
sour odour.” 
* Neutral urine presents this same reaction. 
* For the sake of any one desiring to repeat this experiment, it may be well for me to add a few 
matters of important detail. The estimate of the number of bacteria present in the milk should be 
made within about twenty-four hours of the inoculation of the glass with a purified needle-point dipped 
into milk souring with the pure ferment, and then applied to the milk which is to be inoculated, close 
to the edge of the glass, as distinguished from the central part, from which the drop will be afterwards 
taken. At ordinary temperatures the bacteria will then be found about the period referred to of full 
size and isolated, and therefore easily counted ; whereas if too long a time is allowed to pass after inocu- 
lation, they will be found too numerous to be easily counted, and often of small size and in groups com- 
posed of an indefinite number. Further, if the bacteria are counted in one drop of the milk, and another 
drop is taken for dilution and inoculation, two great inconveniences will result. First, there will be 
no security that the two drops contained the same proportions of bacteria; and, in the second place, 
the knowledge that the bacteria are in process of multiplication with great speed (doubling their numbers 
in about an hour) leads to most undesirable hurry in the microscopic examination and the subsequent 
