540 FRAGMENTS OF SCIENCE. 
Not only did they discover the alcoholic ferment of yeast, 
but they had to exercise a wise selection in picking it out 
from others, and giving it special prominence. Place an 
old boot in a moist place, or expose common paste or a pot 
of jam to the air; it soon becomes coated with a blue-green 
mold, which is nothing else than the fructification of a 
little plant called Penicillium glaucum. Do not imagine 
that the mold has sprung spontaneously from boot, or 
paste, or jam; its germs, which are abundant in the 
air, have been sown, and have germinated, in as legal and 
legitimate a way as thistle-seeds wafted by the wind to a 
proper soil. Let the minute spores of Penicillinm be sown 
in a fermentable liquid, which has been previously so 
boiled as to kill all other spores or seeds which it may con- 
tain; let pure air have free access to the mixture; the 
Penicillinm will grow rapidly, striking long filaments inta 
the liquid, and fructifying at its surface. Test the 
infusion at various stages of the plant's growth, you will 
never find in it a trace of alcohol. But forcibly submerge 
the little plant, push it down deep into the liquid, where 
the quantity of free oxygen that can reach it is insufficient 
for its needs, it immediately begins to act as a ferment, 
supplying itself with oxygen by the decomposition of the 
sugar, and producing alcohol as one of the results of the 
decomposition. Many other low microscopic plants act in 
a similar manner. In aerated liquids they flourish without 
any production of alcohol, but cut off from free oxygen 
they act as ferments, producing alcohol exactly as the real 
alcoholic leaven produces it, only less copiously. For the 
right apprehension of all these facts we are indebted to 
Pasteur. 
In the cases hitherto considered, the fermentation is 
proved to be the invariable correlative of life, being pro- 
duced by organisms foreign to the fermentable substance. 
But the substance itself may also have within it, to some 
extent, the motive power of fermentation. The yeast- 
plant, as we have learned, is an assemblage of living cells; 
but so at bottom, as shown by Schleiden and Schwann, 
are all living organisms. Cherries, apples, peaches, pears, 
plums, and grapes, for example, are composed of cells, 
each of which is a living unit. And here I have to direct 
your attention to a point of extreme interest. In 1821, 
the celebrated French chemist, B6ra.rd., established the 
