d 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,
being apparently competent to propagate indefinitely its own decay.
Why should not a bit of rotten malaria act 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 organism, 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 independently 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 supplied solely with calcined air, putrefaction
never sets in. Putrefaction, therefore, he affirmed to be caused, not
by the air, but by something which could be destroyed by a
sufficiently high temperature. The results of Schwann 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 fermentation. Pasteur, however, proved the real
'ferments,' mediate or immediate, to be organised beings which find in
the reputed ferments their necessary food.

Side by side with these researches and discoveries, and fortified by
them and others, has run the germ theory of epidemic disease. The
notion was expressed by Kircher, and favoured by Linnaeus, that
epidemic diseases may be due to germs which float in the atmosphere,
enter the body, and produce disturbance by the development within the
body of parasitic life. The strength of this theory consists in the
perfect parallelism of the phenomena of contagious disease with those
of life. As a planted acorn gives birth to an oak, competent to
produce a whole crop of acorns, each gifted with the power of
reproducing its parent tree; and as thus from a single seedling a
whole forest may spring; so, it is contended, these epidemic