To ascertain whether the changes effected in nitrous oxide by the
circulating blood acting through the moist coats of the pulmonary
veins of living animals, were highly analogous to those produced in
it by fluid venous blood removed from the vessels, I found extremely
difficult.
I have before observed, that when animals are made to respire nitrous
oxide, a certain absorption of the gas always takes place; but the
smaller animals, the only ones that can be experimented upon in the
mercurial apparatus, die in nitrous oxide so speedily and occasion so
slight a diminution of gas, that I judged it useless to attempt to
analise the residuum of their respiration, which supports flame as well
as pure nitrous oxide, and is chiefly absorbable by water.
In the infancy of my researches, I often respired nitrous oxide in a
large glass bell, furnished with a breathing tube and stop-cock, and
poised in water saturated with the gas.
In two or three experiments in which the nostrils being closed after
the exhaustion of the lungs, the gas was inspired from the bell and
respired into it, a considerable diminution was perceived, and by the
test of lime water some carbonic acid appeared to have been formed; but
on account of the absorption of this carbonic acid by the impregnated
water, and the liberation of nitrous oxide from it, it was impossible
to determine with the least accuracy, the quantities of products after
respiration.
About this time likewise, I often examined the residuum of nitrous
oxide, after it had been respired in silk bags. In these experiments
when the gas had been breathed for a long time, a considerable
diminution of it was observed, and the remainder extinguished flame and
gave a very slight diminution with nitrous gas. But the great quantity
of this remainder as well as other phænomena, convinced me that
though the oiled silk was apparently air-tight when dry, under slight
pressure, yet during the action of respiration, the moist and warm gas
expired, penetrated through it, whilst common air entered through the
wetted surface.
To ascertain accurately, the changes effected in nitrous oxide by
respiration, I was obliged to make use of the large mercurial airholder
mentioned in Research I. of the capacity of 200 cubic inches. The upper
cylinder of it was accurately balanced so as to be constantly under the
pressure of the atmosphere. To an aperture in it, a stop-cock having
a very large orifice was adapted, curved and flattened at its upper
extremity, so as to form an air-tight mouth-piece.
By accurately closing the nose, and bringing the lips tight on the
mouth-piece, after a few trials I was able to breathe oxygene or common
air in this machine for two minutes or two minutes and half, without
any other uneasy feeling than that produced by the inclination of the
neck and chest towards the cylinder. The power of uniformly exhausting
the lungs and fauces to the same extent, I did not acquire till after
many experiments. At last, by preserving exactly the same posture
after exhaustion of the lungs before the inspiration of the gas to be
experimented upon, and during its compleat expiration, I found that I
could always retain nearly the same quantity of gas in the bronchial
vessels and fauces; the difference in the volume expired at different
times, never amounting to a cubic inch and half.
By connecting the conducting pipe of the mercurial airholder, during
the respiration of the gas, with a small trough of mercury by means of
a curved tube, it became a perfect and excellent breathing machine.
For by exerting a certain pressure on the airholding cylinder, it was
easy to throw a quantity of gas after every inspiration or expiration,
into tubes filled with mercury standing in the trough. In these tubes
it could be accurately analised, and thus the changes taking place at
different periods of the process ascertained.
Whenever I breathed pure nitrous oxide in the mercurial airholder,
after a compleat voluntary exhaustion of my lungs, the pleasurable
delirium was very rapidly produced, and being obliged to stoop on the
cylinder, the determination of blood to my head from the increased
arterial action in less than a minute became so great, as often to
deprive me of voluntary power over the muscles of the mouth. Hence, I
could never rely on the accuracy of any experiment, in which the gas
had been respired for more than three quarters of a minute.
I was able to respire the gas with great accuracy for more than half
a minute; it at first, rather increasing than diminishing the power
of volition; but even in this short time, very strong sensations
were always produced, with sense of fulness about the head, somewhat
alarming; a feeling which hardly ever occurs to me when the gas is
breathed in the natural posture.
In all the numerous experiments that I made on the respiration of
nitrous oxide in this way, a very considerable diminution of gas always
took place; and the diminution was generally apparently greater to the
eye during the first four or five inspirations.
The residual gas of an experiment was always examined in the following
manner. After being transferred through mercury into a graduated
cylinder, a small quantity of concentrated solution of caustic potash
was introduced to it, and suffered to remain in contact with it for
some hours; the diminution was then noted, and the quantity of gas
absorbed by the potash, judged to be carbonic acid. To the remainder,
twice its bulk of pure water was admitted. After agitation and rest for
four or five hours, the absorption by this was noticed, and the gas
absorbed considered as nitrous oxide. The residual unabsorbable gas
was mingled over water with twice its bulk of nitrous gas; and by this
means, its composition, whether it consisted wholly of nitrogene, or of
nitrogene mingled with small quantities of oxygene, ascertained.
From a number of experiments made at different times on the respiration
of nitrous oxide, I select the following as the most accurate.
E. 1. At temperature 54°, I breathed 102 cubic inches of nitrous oxide,
which contained near ¹/₅₀ common air, for about half a minute, seven
inspirations and seven expirations being made. After every expiration,
an evident diminution of gas was perceived; and when the last full
expiration was made, it filled a space equal to 62 cubic inches.