(I
generally do it with a finger of the hand in which I hold the phial) for
otherwise the inflammable air will continue burning, though invisibly in
the day time, till the whole be consumed.

SECTION IV.

_Of AIR infected with ANIMAL RESPIRATION, or PUTREFACTION._

That candles will burn only a certain time, in a given quantity of air
is a fact not better known, than it is that animals can live only a
certain time in it; but the cause of the death of the animal is not
better known than that of the extinction of flame in the same
circumstances; and when once any quantity of air has been rendered
noxious by animals breathing in it as long as they could, I do not know
that any methods have been discovered of rendering it fit for breathing
again. It is evident, however, that there must be some provision in
nature for this purpose, as well as for that of rendering the air fit
for sustaining flame; for without it the whole mass of the atmosphere
would, in time, become unfit for the purpose of animal life; and yet
there is no reason to think that it is, at present, at all less fit for
respiration than it has ever been. I flatter myself, however, that I
have hit upon two of the methods employed by nature for this great
purpose. How many others there may be, I cannot tell.

When animals die upon being put into air in which other animals have
died, after breathing in it as long as they could, it is plain that the
cause of their death is not the want of any _pabulum vitae,_ which has
been supposed to be contained in the air, but on account of the air
being impregnated with something stimulating to their lungs; for they
almost always die in convulsions, and are sometimes affected so
suddenly, that they are irrecoverable after a single inspiration, though
they be withdrawn immediately, and every method has been taken to bring
them to life again. They are affected in the same manner, when they are
killed in any other kind of noxious air that I have tried, viz. fixed
air, inflammable air, air filled with the fumes of brimstone, infected
with putrid matter, in which a mixture of iron filings and brimstone has
stood, or in which charcoal has been burned, or metals calcined, or in
nitrous air, &c.

As it is known that _convulsions_ weaken, and exhaust the vital powers,
much more than the most vigorous _voluntary_ action of the muscles,
perhaps these universal convulsions may exhaust the whole of what we may
call the _vis vi