ave undergone
at the same time some chemical change, as in agitating milk or wine, till
they become sour.

Besides the supposed production of phosphoric acid, and change of colour of
the blood, and the production of carbonic acid, there would appear to be
something of a more subtile nature perpetually acquired from the
atmosphere; which is too fine to be long contained in animal vessels, and
therefore requires perpetual renovation; and without which life cannot
continue longer than a minute or two; this ethereal fluid is probably
secreted from the blood by the brain, and perpetually dissipated in the
actions of the muscles and organs of sense.

That the blood acquires something from the air, which is immediately
necessary to life, appears from an experiment of Dr. Hare (Philos.
Transact. abridged, Vol. III. p. 239.) who found, "that birds, mice, &c.
would live as long again in a vessel, where he had crowded in double the
quantity of air by a condensing engine, than they did when confined in air
of the common density." Whereas if some kind of deleterious vapour only was
exhaled from the blood in respiration; the air, when condensed into half
its compass, could not be supposed to receive so much of it.

II. Sir Edward Hulse, a physician of reputation at the beginning of the
present century, was of opinion, that the placenta was a respiratory organ,
like the gills of fish; and not an organ to supply nutriment to the foetus;
as mentioned in Derham's Physico-theology. Many other physicians seem to
have espoused the same opinion, as noticed by Haller. Elem. Physiologiae, T.
1. Dr. Gipson published a defence of this theory in the Medical Essays of
Edinburgh, Vol. I. and II. which doctrine is there controverted at large by
the late Alexander Monro; and since that time the general opinion has been,
that the placenta is an organ of nutrition only, owing perhaps rather to
the authority of so great a name, than to the validity of the arguments
adduced in its support. The subject has lately been resumed by Dr. James
Jeffray, and by Dr. Forester French, in their inaugural dissertations at
Edinburgh and at Cambridge; who have defended the contrary opinion in an
able and ingenious manner; and from whose Theses I have extracted many of
the following remarks.

First, by the late discoveries of Dr. Priestley, M. Lavoisier, and other
philosophers, it appears, that the basis of atmospherical air, called
oxygene, is received by the blood