324 A FURTHER CONTRIBUTION TO THE 
of all the fermentative changes to which milk is liable, that which results in the 
rapid evolution of lactic acid, and consequent precipitation of the caseine in the 
form of curd, a change which was attributed by Pasteur, so early as 1857, to 
the operation of a special organism! The frequency of this change in milk 
does not, however, appear to depend on specially extensive dissemination of 
the ferment, but rather upon the circumstance that the organism which we are 
about to study, when it does gain access to milk, takes the precedence of others 
in development, and that dairies being places in which this particular ferment 
abounds, the milk supplied from them is sure to contain it, as they are at present 
managed. For it is a remarkable fact, and one well worthy of the consideration 
of the dairyman, that while milk supplied for domestic use will turn sour in 
summer weather within twenty-four hours, yet of all the many instances in which 
I have observed alterations in milk caused by organisms introduced through 
atmospheric exposure, in no single case did the true lactic-acid fermentation 
occur. Some organisms have given rise to a primary alkaline alteration, strong 
or feeble, some have been neutral in their effects, while others have produced 
an acid condition indeed, but only feeble and slowly developed. 
It seemed worth while before closing this investigation, in which fermenta- 
tive changes in milk had occupied a prominent position, to apply our method 
of inquiry to the most frequent and therefore the most interesting of them all. 
Accordingly on the 14th of last month, August, I obtained from a dairy near 
Edinburgh, pervaded with the usual sour smell, about a pint of milk said to 
have been taken from the cow four hours previously and tasting perfectly fresh, 
the dairy woman bailing it out with a tin vessel from the pan in which it stood 
into a clean glass bottle which I had provided. One hour later about ten ounces 
were introduced into a flask purified by heat, and were subjected to the tem- 
perature of 212° Fahr. for three-quarters of an hour, the arrangements being such 
as have been fully described above, see p. 310, and on the following day four 
experimental glasses were charged each with about half an ounce of the milk 
by means of a permanent syphon (see above). The first milk that came from 
the syphon, received into another glass, had the taste of perfectly fresh boiled 
milk, it purpled both blue and red litmus paper, and exhibited under the micro- 
scope nothing but milk globules of all sizes including extreme minuteness. 
Meanwhile, the milk remaining in the bottle had undergone the usual change. 
At noon, twenty-three hours after it was taken from the cow, it tasted distinctly 
sour though still fluid, and sharply reddened blue litmus, and on microscopic 
examination motionless bacteria were seen in considerable numbers, of soft or 
* “Mémoire sur la Fermentation appelée Lactique,’ Annales de Chimie et de Physique, 3™° série, 
tome li, 1858. 
