at and
the air absorbed, we must carefully mark upon the bell-glass, with a
diamond, the height of the mercury, both before and after the
experiment[9]. After this, the syphon (GH, Pl. IV. fig. 3.) guarded, as
before, with a bit of paper, to prevent its filling with mercury, is to
be introduced under the bell-glass, having the thumb placed upon the
extremity, G, of the syphon, to regulate the passage of the air; and by
this means the air is gradually admitted, so as to let the mercury fall
to its level. This being done, the bell-glass is to be carefully
removed, the globules of melted iron contained in the cup, and those
which have been scattered about, and swim upon the mercury, are to be
accurately collected, and the whole is to be weighed. The iron will be
found in that state called _martial ethiops_ by the old chemists,
possessing a degree of metallic brilliancy, very friable, and readily
reducible into powder, under the hammer, or with a pestle and mortar. If
the experiment has succeeded well, from 100 grains of iron will be
obtained 135 or 136 grains of ethiops, which is an augmentation of 35
per cent.

If all the attention has been paid to this experiment which it deserves,
the air will be found diminished in weight exactly equal to what the
iron has gained. Having therefore burnt 100 grains of iron, which has
acquired an additional weight of 35 grains, the diminution of air will
be found exactly 70 cubical inches; and it will be found, in the sequel,
that the weight of vital air is pretty nearly half a grain for each
cubical inch; so that, in effect, the augmentation of weight in the one
exactly coincides with the loss of it in the other.

I shall observe here, once for all, that, in every experiment of this
kind, the pressure and temperature of the air, both before and after the
experiment, must be reduced, by calculation, to a common standard of 10
deg. (54.5 deg.) of the thermometer, and 28 inches of the barometer.
Towards the end of this work, the manner of performing this very
necessary reduction will be found accurately detailed.

If it be required to examine the nature of the air which remains after
this experiment, we must operate in a somewhat different manner. After
the combustion is finished, and the vessels have cooled, we first take
out the cup, and the burnt iron, by introducing the hand through the
quicksilver, under the bell-glass; we next introduce some solution of
potash, or caustic alkali,