GERM THEORY OF FERMENTATIVE CHANGES 329 
apparent thickness as black as pitch, showing out in a glaring contrast to the 
white milk. The black material did not appear to undergo any increase in 
the course of the day or at any subsequent period. But there was a peculiar 
sickly, almost putrefactive, smell mingled with the sour odour of the air in the 
glass shade in the course of the next twenty-four hours, though this afterwards 
passed off, and by the time that curdling was complete a pure smell of sour 
milk was alone perceptible. On the 26th I turned out the curd to investigate 
the black substance. I found it adhering firmly to the bottom of the vessel 
so that it could be completely cleansed of the curd with a camel’s-hair brush 
without being detached ; and when I picked it out with a knife its lower surface 
had a brilliant polish, corresponding to that of the glass. It constituted a tough 
scale, between horny and leathery in consistence, and its upper surface presented 
numerous smooth round depressions with intervening ridges ; and it was plain 
that the pigment had been precipitated in the form of a heavy liquid, the particles 
of which had coalesced at the bottom of the vessel and afterwards solidified. 
The intensity of the colour was strikingly brought out by microscopic examina- 
tion under my highest power, when even parts of extreme tenuity, as at g, 
Plate XIII, distinctly showed the sepia tint of the mass. These very thin parts 
also afforded the opportunity of ascertaining that the substance was perfectly 
homogeneous and structureless. In other words, the dark substance was not 
a coloured organism, but a pigment formed from the milk as the result of the 
growth of an organism in it. The small amount of the material at my disposal 
permitted me to ascertain only that it was insoluble in water, spirit of wine, 
anhydrous ether, and a strong solution of caustic potash, both in the cold and 
boiling states of these fluids, and was also unaffected by cold nitric acid, but 
was dissolved by boiling nitric acid, to which it communicated a yellow colour. 
Heated in a glass tube with access of air it burnt without fusion, leaving a 
white ash. 
The question of course presents itself, what was the cause of this remark- 
able formation of pigment from the milk ? That it was induced by an organism 
introduced into the milk we cannot doubt. But was that organism the same 
bacterium that in the former glass of boiled milk, as in the original stock of 
unboiled milk, produced only the lactic-acid fermentation, but altered in function 
while modified in form by its residence in the other media, or was it some other 
species, some ‘ pigment bacterium’, to use Professor Cohn’s expression, coexist- 
ing with the lactic-acid ferment ? Before discussing this question I must direct 
attention again to the glassof Pasteur’s solution from which the second urine- 
glass was inoculated. It may be remembered that at the time of that inocu- 
lation there was already present a dingy or dirty aspect about that glass such 
