56 Pasteur and his Work, 
organism consists of minute rods, separate, or united in chains of 
two, three, or more, which reproduce themselves by division, and 
which have the power of movement — gliding in an undulating 
manner, and breaking themselves off from each other by this 
motile faculty. They can be grown, like the other ferments, in 
fluids containing fermentable substances, in which they will mul- 
tiply to an almost indefinite extent, their increase marking the 
Fig. 2. — Clostridium Butyricum, or Bacillus Butyricus (the Butyric 
Ferment^ 
Some of the Bpindle-ehaped lorma include an oval spore. 
progress of the butyric fermentation. In studying this organism 
or vibrio, Pasteur came upon a new and altogether startling 
peculiarity of these ferments — ^that they can not only multiply 
freely without air, but that the presence of air deprives them of 
life, and stops the fermentation to which they give rise. A 
stream of pure carbonic acid, so deadly to animals, may be 
assed through the fluid in which they are growing, without 
affecting them ; but if a current of atmospheric air be sub- 
stituted for the acid for a brief space, the organisms subside 
motionless to the bottom, and fermentation is at once arrested. 
The question as to the way in which the ferments induced the 
phenomena of fermentation had to be answered, and in attempt- 
ing it Pasteur was brought nearer to a solution of the mystery. 
The micro-organisms, like the higher animals, were nourished 
upon suitable pabulum — living upon a portion of the fermentable 
matter ; but while the animal, for a given weight of nutritive 
matter ingested, assimilates a certain quantity, the "microbe" 
(as Sedillot named these minute plants), in consuming some of 
the matter, decomposes a quantity far in excess of its own weight. 
The " must," or sweet wort of beer or wine, when placed in vats 
or barrels to produce these fluids, will undergo fermentation 
when yeast has been purposely added, or when the ferment- 
germs have been accidentally introduced; and the vital actions, 
of the germs — multiplication, and increase in weight and 
volume — go on entirely independently of the free oxygen of the 
air, or of that in the " must." In the immense vats of breweries, 
fermentation disengages quantities of carbonic-acid gas, which 
is so much heavier than the atmosphere, that it rests in a dense 
layer on the surface of the fluid, and completely excludes the 
