posit
the fixed air it contains; for if any lime-water be exposed to it, it
immediately becomes turbid. This is the case, when wax candles, tallow
candles, chips of wood, spirit of wine, ether, and every other substance
which I have yet tried, except brimstone, is burned in a close glass
vessel, standing in lime-water. This precipitation of fixed air (if this
be the case) may be owing to something emitted from the burning bodies,
which has a stronger affinity with the other constituent parts of the
atmosphere[3].

If brimstone be burned in the same circumstances, the lime-water
continues transparent, but still there may have been the same
precipitation of the fixed part of the air; but that, uniting with the
lime and the vitriolic acid, it forms a selenetic salt, which is soluble
in water. Having evaporated a quantity of water thus impregnated, by
burning brimstone a great number of times over it, a whitish powder
remained, which had an acid taste; but repeating the experiment with a
quicker evaporation, the powder had no acidity, but was very much like
chalk. The burning of brimstone but once over a quantity of lime-water,
will affect it in such a manner, that breathing into it will not make it
turbid, which otherwise it always presently does.

Dr. Hales supposed, that by burning brimstone repeatedly in the same
quantity of air, the diminution would continue without end. But this I
have frequently tried, and not found to be the case. Indeed, when the
ignition has been imperfect in the first instance, a second firing of
the same substance will increase the effect of the first, &c. but this
progress soon ceases.

In many cases of the diminution of air, the effect is not immediately
apparent, even when it stands in water; for sometimes the bulk of air
will not be much reduced, till it has passed several times through a
quantity of water, which has thereby a better opportunity of absorbing
that part of the air, which had not been perfectly detatched from the
rest. I have sometimes found a very great reduction of a mass of air, in
consequence of passing but once through cold water. If the air has stood
in quicksilver, the diminution is generally inconsiderable, till it has
undergone this operation, there not being any substance exposed to the
air that could absorb any part of it.

I could not find any considerable alteration in the specific gravity of
the air, in which candles, or brimstone, had burned out. I am satisfi