372 ON THE LACTIC FERMENTATION 
by Pasteur’s solution, does not find in that fluid the materials requisite for its 
nutrition. I also imitated in another glass of Pasteur’s solution the experiment 
of four years before, by inoculating it with urine in which the Bacterium lactis 
was developing actively, but here again, although the specimen was kept twenty- 
one days, no sign whatever of bacteric growth appeared. It was therefore plain 
that the Bacterium lactis is really incapable of growing in pure Pasteur’s solution.t 
Thus I was forced to the conclusion that the appearances which I had 
described in my former communication as due to modifications of the Bacterium 
lactis in urine and Pasteur’s solution had been entirely deceptive, and had been 
occasioned by the accidental accompaniment of other species. And now that 
we know from the results of experiments like those with the little test-tube 
glasses how very numerous are the bacteria and other organisms which really 
infest milk, it is easy to understand how such a confusion may have arisen.” 
Now, however, having good reason to believe that I had got the Bacterium 
Jactis pure and unmixed, I proceeded to perform the experiment which con- 
stitutes the most important feature of this investigation. I have already 
described the mode of procedure when speaking of the means adopted for isolat- 
ing the Bacterium lactis. Its object now was to obtain, if possible, absolute 
proof, which would commend itself to the judgement of all, that the Bacterium 
lactis is really the cause of the lactic fermentation, and no mere accidental 
concomitant of the change. 
* It is right that I should state that the solution which I employed differed from that made accord- 
ing to Pasteur’s formula, in having only half the proportion of sugar, and in having its mineral salts 
derived from wood ashes instead of from the ashes of yeast. It was, however, with solution of precisely 
the same composition that I had worked in the experiments of four years previously. 
* It is not, indeed, altogether easy to see what the precise train of events may have been in the 
experiments of four yearsago. I may suggest the following as possible. First, along with the Bacterium 
/actis with which the first urine-glass was inoculated (see p. 367), was accidentally introduced another 
kind of bacterium, which produced the large spirillum-like appearances. This adventitious organism 
grew freely in the urine, along with the slowly growing and inconspicuous Bacterium lactis. It presented 
considerable varieties of form, some of them closely resembling chains of Bacterium lactis, but it was 
in reality a totally distinct species. Then when the Pasteur’s solution was inoculated from the urine, 
both organisms were again introduced together. Here the spirillum-like organism underwent a 
modification, assuming the characters of an actively moving double bacterium, a transition which was, 
I think, sufficiently traced under the microscope (see Microscopical Journal, loc. cit.). Meanwhile, the 
Bacterium lactis lay dormant in this medium, but retained its vitality, and by accident it happened 
that one or more of these still living individuals, lying perhaps at the surface of the liquid in the speck 
of scum that commonly results from inoculation, was transferred along with the other organism to the 
last urine-glass. Here the latter resumed the spirillum-like form which it had before shown in urine, 
though retaining now for a while in a limited degree the movement it had acquired in Pasteur’s solution. 
The individual or individuals of the Bacterium Jactis introduced into the urine along with it were here 
able to develop again, though slowly, and some of their progeny entered the glass of boiled milk inocu- 
lated from this urine-glass, and in that medium developed with their accustomed rapidity and with 
the usual fermentative effect. It may, perhaps, seem trivial to record this complicated hypothetical 
explanation, but I am induced to do so for the sake of illustrating the excessive care required in order 
to avoid deception when we are dealing with such extremely minute organisms. 
