l
or vegetable putrefaction, when a single inspiration of it would have
instantly killed any other animal. I have frequently tried the
experiment with flies and butterflies. The _aphides_ also will thrive as
well upon plants growing in this kind of air, as in the open air. I
have even been frequently obliged to take plants out of the putrid air
in which they were growing, on purpose to brush away the swarms of these
insects which infected them; and yet so effectually did some of them
conceal themselves, and so fast did they multiply, in these
circumstances, that I could seldom keep the plants quite clear of them.

When air has been freshly and strongly tainted with putrefaction, so as
to smell through the water, sprigs of mint have presently died, upon
being put into it, their leaves turning black; but if they do not die
presently, they thrive in a most surprizing manner. In no other
circumstances have I ever seen vegetation so vigorous as in this kind of
air, which is immediately fatal to animal life. Though these plants have
been crouded in jars filled with this air, every leaf has been full of
life; fresh shoots have branched out in various directions, and have
grown much faster than other similar plants, growing in the same
exposure in common air.

This observation led me to conclude, that plants, instead of affecting
the air in the same manner with animal respiration, reverse the effects
of breathing, and tend to keep the atmosphere sweet and wholesome, when
it is become noxious, in consequence of animals either living and
breathing, or dying and putrefying in it.

In order to ascertain this, I took a quantity of air, made thoroughly
noxious, by mice breathing and dying in it, and divided it into two
parts; one of which I put into a phial immersed in water; and to the
other (which was contained in a glass jar, standing in water) I put a
sprig of mint. This was about the beginning of August 1771, and after
eight or nine days, I found that a mouse lived perfectly well in that
part of the air, in which the sprig of mint had grown, but died the
moment it was put into the other part of the same original quantity of
air; and which I had kept in the very same exposure, but without any
plant growing in it.

This experiment I have several times repeated; sometimes using air in
which animals had breathed and died, and at other times using air,
tainted with vegetable or animal putrefaction; and generally with the
same su