have on the internal air,
with respect to their continued existence in it, is abundantly proved by
the growth of plants in Ward's cases, from the interior of which the
external air is excluded as fully as it possibly can be, without their
being actually sealed: if therefore, any injurious effects result to
plants, from their being cultivated in a close atmosphere, we must seek
for the cause, in some other source, than the plants themselves. If any
noxious qualities exist in the atmosphere of structures, to which the
external air has not free ingress, they must result from some neglect or
ignorance on our part, in suffering extraneous and unwholesome matters to
accumulate in such situations, and there to decompose, and enter into
combination with those gaseous bodies, which form the volume of the
internal atmosphere of our plant structures. The existence of such
extraneous matters, may indeed be traced to various sources; and they may
be present, even when much vigilance is employed to prevent their
accumulation; and therefore, as an inconceivably minute quantity,
inappreciable to the senses, would frequently be sufficient to effect
deterioration, it is possible that these impurities may often originate in
sources which are least of all suspected. The decomposition of organic
matter, whether animal or vegetable, may frequently be the source of
injurious results in this respect; for although this is principally
resolved into those elementary gases, which appear to form the basis of
all created objects, yet there are other matters liberated, which may then
enter into fresh combinations; and either this, or a disproportionate
accumulation, even of these elementary bodies, may reasonably give rise to
serious apprehension, and demand the exercise of discretion, in order to
prevent them from becoming injurious. Besides this, these decomposing
bodies, afford just the very state of things, which appears to be
requisite to call into existence, and developement, a numerous phalanx of
cryptogamic vegetables: not that such matters, can for a moment be
rationally considered to generate, these _cellulares_; but that they
afford a suitable pabulum, and medium of developement for those millions
upon millions of sporules, which we may readily conceive to be dispersed
in the atmosphere; and with which it may be teeming, though from their
buoyancy and minuteness, they may float to us invisibly therein.

The admission of the external air, b