80 
NATURE 
the fungi, it is the fungi which produce the particular form 
of decay. 
When a mildew is growing in the ordinary way in the 
free air on the surface of a liquid containing sugar, or on 
the surface of a plant, it absorbs oxygen from the air, and 
combines the oxygen thus absorbed with carbon, the pro- 
duct of the decomposition of the matter on which it is 
growing, so that by this ordinary process of burning, 
carbon dioxide and water are set free, while at the same 
time putrefaction is kept up in the substance attacked, 
If protein compounds be present, then ammonia, sul- 
phuretted hydrogen, and other substances are likewise set 
at liberty, making the putrefaction more offensive. The 
fungus is, in this case, feeding upon the organic matter, 
and breathing the oxygen just like an animal. It cannot 
decompose carbonic acid if it be freely supplied with this 
gas. Without any other source of carbon, it does not in- 
crease. The relation of fungi to the other substances re- 
quired for their growth is still uncertain, It has been 
supposed, and experiment seems to favour the opinion, 
that fungi can assimilate the nitrogen of ammonia and 
nitric acid, and even that they can absorb and assimilate the 
nitrogen of the air. I should think this very doubtful. It 
would seem most probable that in their relation to their 
surrounding sources of nourishment, their reactions are 
the same as those of animals and of the pale parts of the 
higher plants. 
Pasteur has shown that the same plants which, when 
growing fully exposed to the air and liberally supplied 
with oxygen, produce putrefaction, will, when partially or 
wholly excluded from the air, and deprived of a full sup- 
ply of oxygen, produce fermentation—that is to say, will 
induce and keep up a set of changes resulting in the pro- 
duction, not of carbonic acid and water, but of alcohol, 
or of acetic, butyric, or lactic acid. 
The rationale of this process proposed for acceptance 
by Pasteur is singularly beautiful, and will, if correct, cause 
a great change in our ideas of the vital relations of these 
lower living forms. He believes that ferments are living 
beings with this special property—that they can perform 
all their vital functions without being in contact with 
free oxygen gas; that they can take the oxygen which is 
necessary for their respiration, and for other changes in the 
organic matter upon which they are feeding, from organic 
compounds containing oxygen, such as sugar; that they 
can decompose and burn these, and in doing so induce 
in a large quantity of fermentable material the con- 
version of sugar into alcohol. Pasteur cites the following 
experiment :— 
If we half fill a flask with a fermentable liquid such as a 
solution of sugar, and having taken all care to exclude all 
other germs, sow on its surface some spores of Mycoderma 
vini or Penicillium glaucum, the fungus grows and flour- 
ishes on the surface, feeding on the organic matter in the 
solution, absorbing oxygen from the air, and throwing off 
carbon dioxide. In this case no alcohol is produced. If 
we now shake the flask, the film of fungus sinks through 
the liquid, and for a time there is no further change ; but 
after resting a little, if the temperature be kept up, bubbles 
of carbon dioxide begin to rise from the fungus, which 
continues to grow, although more slowly. Fermentation 
sets in instead of putrefaction, and alcohol is produced in 
sensible quantities. The one great change which has been 
produced in the circumstances of the fungus is that it has — 
now been almost wholly excluded from contact with free _ 
oxygen, while in its former condition it was bathed in it. — 
Upon this change, according to Pasteur, depends its now ~ 
acting as a ferment instead of inducing putrefaction. 
A ferment, then, is a living body which is special in this 
respect; that it is capable of performing all the functions — 
of its life apart from free oxygen; it can assimilate — 
directly oxygenated matters such as sugar, and derive - 
from them the requisite amount of heat, and it further 
can produce the decomposition of a much greater weight of — 
fermentable matter than the weight of the ferment in action, d 
Pasteur has found that ferments such as yeast lose their — 
fermenting power— that is to say, the amount of organic — 
matter decomposed diminishes and approaches the weight 
of the ferment employed—exactly in proportion to the 
amount of free oxygen supplied. 
Pasteur has also shown, and this is one of the most 
curious results of his investigations, that the same fungus 
does not incite or maintain the alcholic, the acetic acid, — 
the lactic acid, or the butyric acid fermentations ; but 
that these changes are produced by different species, — 
nearly allied but distinguishable from one another under — 
the microscope ; the specific differences between them 
extending to this strange difference in their powers of — 
nutrition or respiration which induces different reactions — 
in a fermentable fluid. 
In the course of the foregoing remarks we have digressed — 
widely from our text, the ripening and rotting “ Duchesse 
dAngouléme” pears ; but, before concluding, let us fora — 
moment recur to them, and see how far the facts and — 
theories which I have brought before you are applicable — 
to the considerations from which we started—their ripening — 
and their decay. This ripe pear, during its early growth, - 
was green. The cells in its outer layer contained chloro- — 
phyll, and contributed their quota to the shaking asunder — 
of the elements of carbon dioxide and water under the © 
influence of light—to the nutrition of the pear tree. Its — 
inner pale cells grew, amply supplied with food from the — 
elaborated sap, and with oxygen suspended in the perco- 
lating fluids and passing through the many ducts. Thus — 
at that period growth was going oa, and neither fermen- 
tation nor putrefaction. Sugar, starch, and various sub- 
stances were then laid down in the ce'ls, and when the - 
pear had acquired its full growth, the connection with — 
the tree was cut off, but the surface of the fruit remained — 
still freely exposed o the air. A considerable quantity — 
of sugar is now decomposed in the interior of the fruit, 
and the result is the production of a trace of alcohol and — 
certain ethers—the development of the flavour of the 
pear ; but shortly the outside softens by the ordinary pro- — 
duction of water and carbon dioxide in contact with the — 
oxygen of the air, the pear loses flavour again, and — 
commences to decay. M. Bérard has shown that ifa — 
ripening fruit be placed in an atmosphere of carbon diox- 
ide no such softening occurs. The changes are much less 
rapid, the inner cells of the pear act as a ferment, and — 
while carbon dioxide is still given off it is now at the 
expense of the sugar, and a large quantity of alcohol is 
the result. M. Pasteur tried this experiment with four 
dozen “ Monsieur” plums taken nearly ripe from the tree; _ 
twenty-four of these were placed in an atmosphere of 
carbon dioxide, and after several days, during which time - 
