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diseases generally were propagated by a kind of malaria,
which consisted of organic matter in a state of motor-decay;
that when such matter was taken into the body through
the lungs, skin, or stomach, it had the power of spreading
there the destroying process which had attacked itself.
Such a power was visibly exerted in the case of yeast. A
little leaven was seen to leaven the whole lump, a mere
speck of matter in this supposed state of decomposition be-
ing apparently competent to propagate indefinitely its own
decay. Why should not a bit of rotten malaria work in a
similar manner within the human frame ? In 1836 a very
wonderful reply was given to this question. In that year
Cagniard de la Tour discovered the yeast-plant, a living or-
ganis‘m, which, when placed in a proper medium, feeds, grows,
and reproduces itself, and in this way carries on the process
which we name fermentation. By this striking discovery
fermentation was connected with organic growth.
Schwann, of Berlin, discovered the yeast-plant inde-
pendently about. the same time; and in February, 1837, he
also announced the important result that, when a decoction
of meat is effectually screened from ordinary air, and sup-
plied solely with calcined air, putrefaction never sets in.
Putrefaction, therefore, he aflirmed to be caused by some-
thing derived from the air, which something could be de-
stroyed by a sufficiently high temperature. The results of
Swann were confirmed by the independent experiments of
Helmholtz, Ure, and Pasteur, while other methods, pursued
by Schultze and by Schroeder and Dusch, led to the same
result. But as regards fermentation, the minds of chemists,
influenced probably by the great authority of Gay-Lussac,
fell back upon the old notion of matter in a state of decay.
It was not the living yeast-plant, but the dead or dying
parts of it, which, assailed by oxygen, produced the fer-
mentation. This notion was finally exploded by Pasteur.
He proved that the so-called “ ferments ” are not such ;

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