st analysis will not discover in either of them a particle
of iron, while the red globules thus separated will be found to
contain a considerable quantity of this metal.

That the red colour of the blood depends upon iron, appears likewise
from the experiments of Menghini, which show, that the blood of
persons who have been taking chalybeate medicines for some time, is
much more florid that it is naturally; the same is agreeable to my
own observation. A late analysis, by Fourcroy, has likewise proved,
that the red colour of the blood resides in the iron; but, though the
red colour of the blood may reside in the iron which it contains, we
shall find that this colour is likewise connected with oxidation.

If the dark coloured blood, drawn from the veins, be put under a
vessel containing oxygen gas, its surface will immediately become
florid, while the bulk of the gas will be diminished. Mr. Hewson
enclosed a portion of a vein between two ligatures, and injected into
it a quantity of oxygen gas; the blood, which was before dark
coloured, instantly assumed the hue of arterial blood. Thuvenal put a
quantity of arterial blood under the receiver of an air pump; on
exhausting the air it became of the dark colour of venous blood; on
readmitting the air, it became again florid. He put it under a
receiver filled with oxygen gas, and found the florid colour much
increased.

Dr. Priestly exposed the blood of a sheep successively to oxygen gas,
atmospheric air, and carbonic acid gas; and found, that in oxygen gas
its colour became very florid, less so in atmospheric air, and in
carbonic acid gas it became quite black. He filled a bladder with
venous blood, and exposed it to oxygen gas; the surface in contact
with the bladder immediately became florid, while the interior parts
remained dark coloured.

All these facts prove, that the red colour which the blood acquires
in the lungs, is owing to the oxygen, which probably combines with
it, and the last mentioned fact shows, that oxygen will act on the
blood, even though a membrane similar to the bladder, be interposed
between them.

The same effect, probably, takes place in the lungs; the blood is
circulated through that organ by a number of fine capillary arteries;
and it is probable that the oxygen acts upon the blood through the
membranes of these arteries, in the same manner that it does through
the bladder.

In short, it seems likely, that the blood, during its circulation