tae_ at once, at least that the lungs may be rendered
absolutely incapable of action, till the animal be suffocated, or be
irrecoverable for want of respiration.

If a mouse (which is an animal that I have commonly made use of for the
purpose of these experiments) can stand the first shock of this
stimulus, or has been habituated to it by degrees, it will live a
considerable time in air in which other mice will die instantaneously. I
have frequently found that when a number of mice have been confined in a
given quantity of air, less than half the time that they have actually
lived in it, a fresh mouse being introduced to them has been instantly
thrown into convulsions, and died. It is evident, therefore, that if the
experiment of the Black Hole were to be repeated, a man would stand the
better chance of surviving it, who should enter at the first, than at
the last hour.

I have also observed, that young mice will always live much longer than
old ones, or than those which are full grown, when they are confined in
the same quantity of air. I have sometimes known a young mouse to live
six hours in the same circumstances in which an old mouse has not lived
one. On these accounts, experiments with mice, and, for the same reason,
no doubt, with other animals also, have a considerable degree of
uncertainty attending them; and therefore, it is necessary to repeat
them frequently, before the result can be absolutely depended upon. But
every person of feeling will rejoice with me in the discovery of
_nitrous air_, to be mentioned hereafter, which supersedes many
experiments with the respiration of animals, being a much more accurate
test of the purity of air.

The discovery of the provision in nature for restoring air, which has
been injured by the respiration of animals, having long appeared to me
to be one of the most important problems in natural philosophy, I have
tried a great variety of schemes in order to effect it. In these my
guide has generally been to consider the influences to which the
atmosphere is, in fact, exposed; and, as some of my unsuccessful trials
may be of use to those who are disposed to take pains in the farther
investigation of this subject, I shall mention the principal of them.

The noxious effluvium with which air is loaded by animal respiration, is
not absorbed by standing, without agitation; in fresh or salt water. I
have kept it many months in fresh water, when, instead of being
meliorated, it