FERMENTATION. 543 
proved an ignis fatuus instead of & pharos to some of his 
followers. 
I have said that our air is full of the germs of ferments 
differing from the alcoholic leaven, and sometimes seriously 
interfering with the latter. They are the weeds of this 
microscopic garden which often overshadow and choke the 
flowers. Let us take an illustrative case. Expose milk to 
the air. It will, after a time, turn sour, separating like 
hlood into clot and serum. Place a drop of this sour milk 
under a powerful microscope and watch it closely. You 
see the minute butter-globules animated by that curious 
quivering motion called the Brownian motion. But let 
not this attract your attention too much, for it is another 
motion that we have now to seek. Here and there vou observe 
a greater disturbance than ordinary among the globules; 
keep your eye upon the place of tumult, and you will 
probably see emerging from it a long eel-like organism, 
tossing the globules aside and wriggling more or less 
rapidly across the field of the microscope. Familiar with 
one sample of this organism, which from its motions receives 
the name of vibrio, you soon detect numbers of them. It 
is these organisms, and other analogous though apparently 
motionless ones, which by decomposing the milk render it 
sour and putrid. They are the lactic and putrid ferments, 
as the yeast-plant is the alcoholic ferment of sugar. Keep 
them and their germs out of your milk and it will continue 
sweet. But milk may become putrid without becoming 
sour. Examine such putrid milk microscopically, and you 
find it swarming with shorter organisms, sometimes asso- 
ciated with the vibrios, sometimes alone, and often mani- 
festing a wonderful alacrity of motion. Keep these 
organisms and their germs out of your milk and it will 
never putrefy. Expose a mutton-chop to the air and keep 
it moist; in summer weather it soon stinks. Place a drop 
of the juice of the fetid chop under a powerful microscope; 
it is seen swarming with organisms resembling those in 
the putrid milk. These organisms, which receive the 
common name of bacteria,* are the agents of all putre- 
faction. Keep them and their germs from your meat and 
* Doubtless organisms exhibiting grave specific differences are 
grouped together under this common name. 
