Observations on respiration, and the use of the blood / by Joseph Priestley ... ; read at the Royal Society, Jan. 25, 1776

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OBSERVATIONS

O N

RESPIRATION,

AND THE USE OF

THE B L O O D.

BY JOSEPH PRIESTLEY, LL.D, F.R.S.

Head at the R O Y A L SOCIETY, Jam 25, *77<S.

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LONDON:

Printed in the Year MDCCLXXVI.

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OBSERVATIONS, &c.

THERE is, perhaps, no fubjeil in phyilology, and
very few in philofophy in general, that has en-
gaged more attention than that of the ufe of refpiration.
It is evident, that without breathing moft animals would
prefently die ; and it is alfo well known, that the fame air
will not long anfwer the purpofe : for if it has been fre-
quently refpired, the breathing of it is as fatal as the
total deprivation of air. But by what property it is, that
air contributes to the fupport of animal life; and why
air that has been much breathed will no more anfwer
the purpofe, feems not to have been difcovered by any of
the many philofophers and phyficians who have pro-
feffedly written upon the fubje6t ; and it might have con-
tinued to elude all direEl inveftigation, when it difcovered
itfelf, without any trouble or thought, in the courfe of
my refearches into the properties of different kinds of
air, which had at firft quite another objeit.

In thefe experiments it clearly appeared, that refpiration
isixpblogijtic procefs, affedting air in the very fame manner

A 2 as

4 Dr. Priestley's Obfervations on

as every other phlogiftic procefs (viz. putrefaction, the
effervefcence of iron-filings and brimftone, or the calcina-
tion of metal s,&c.) affe&s it ; diminifhing the quantity of it
in a certain proportion, leflening its fpecific gravity, and
rendering it unfit for refpiration or inflammation, bvit
leaving it in a ftate capable of being reftored to a tolerable
degree of purity by agitation in water, &x. Having dis-
covered this, I concluded, as maybe feen Phil. Tranf. vol.
LXII. p. 1 8 7 . and Obfervations upon Air^ vol. I. p. 7 8 . 2 7 7 .
that the ufe of the lungs is to carry off a putrid effluvium^.
or to difcharge that phlogifton, which had been taken
into the fyftem with the aliment, and was become, as it
were, effete ; the air that is refpired ferving as -zmenjlruum
for that purpofe.

What I th en concluded to be the ufe of refpiration in ge-
neral, I have now, I think, proved to be effected by means
of the bloody in confequence of its coming fo nearly into
contact with the air in the lungs ; the blood appearing to
be a fluid wonderfully formed to imbibe, and part with,
that principle which the chemifts call phlogifton, and
changing its colour in confequence of being charged with
it, or being freed from it ; and affecting air in the very
fame manner, both out of the body and in the lungs ; and
even notwithftanding the interpofition of various fub-
ftances, which prevent its coming into immediate con-
tail with the air.

As it may not be unpleafing or unufeful, I fhall,
before I relate my own experiments, briefly recite the
principal of the opinions which have been held con-
cerning

Refpiration, and the Ufe of the Blood.. 5;

eerning the ufe of refpiration, from haller's excellent
Syjiem of Phyjiology, and fome others of the molt eminent
writers upon that fubjedt.

Hippocrates reckoned air among the aliments of the
body. But it was more generally the opinion of the an-
cients, that, there being a kind of vital fire kept up in the
heart, the heat of the blood was tempered in the lungs.
galen alfo fuppofes, that there was fomething equiva-
lent to a fire conftantly kept up in the heart; and that
the chief ufe of the lungs was to carry off fuch vapours
as were equivalent to fmoke thrown off from that fire..
haller, vol. III. p. 354. cartesius maintained the
fame vital fire in the heart, fuppoling that air was ne—
ceffary for cooling and condenfing the blood. Ibid. p. 343..

Of the more modern phyfiologifts, fome have thought;
that the air itfelf is taken into the lungs ; others, that it is
only fomething extracted from the air, as the more fubtle
parts of that fluid, an ether, or aerial nitre; while others
fuppofe it to be the air itfelf, but diffolved in water, and
therefore in an unelaftic flate, ibid. p. 3 2 1 .

Moft of thofe who think that air is taken into the
blood fuppofe it to be taken in by the lungs, ibid. p„.
330. Some fuppofe, that the effect of the admiffion
of this air into the blood is a fermentation, p. 332.
Others fuppofe, that it aits by its fpring, preventing
the too clofe contadt of the globules, and thereby pre-
ferring its fluidity, inteftine motion, and heat, ibid, ber-
tier fuppofed, that the circulation of the blood was, in
a great meafure, owing to the admiffion of air into it.
van helmont afcribed the volatility of. the fixed ele-
ments

6 Dr. Priestley's Obfervations on

merits in the food to this air, p. 336.; and stevenson
thought, that the air which had circulated in the blood,
and which had heated the blood too much, was exhaled
by the lungs, p. 355.

Others fay, that the air itfelf is not admitted into the
hlood, but only fome adtive, fpirituous, and ethereal par-
ticles ; that this vital fpirit paffes from the lungs to the
heart and arteries, and at length becomes the animal fpi-
xits, which are by this means generated from the air, p.
333. Others, who do not admit that the animal fpirits
are derived from the air, flill fay that fome other vital
principle comes from thence. This vital principle mal-
pighius fuppofes to be a faline vapour; lister, a hot,
inflammable, fulphureous fpirit; vieussenius, a volatile
acid fait, which keeps up the fermentation of the blood ;
and bryan robinson, the aerial acid, which preferves the
blood from putrefadtion; preferves alfo its denfity, and
ftrengthens the animal fibres. For this reafon he fup-
pofes it is that we feel ourfelves refrefhed in cold air, as
it abounds with a more plentiful acid quality, p. 334-
They who fuppofe that nitre is taken from the air into
the blood, afcribe to that principle its fermentation, its
heat, and its denfity, p. 334.

It is a received opinion, that one ufe of the lungs is to
attenuate the blood, p. 359; and malpighius adds,
that by this means, the different particles of the blood
become thoroughly mixed together; while others think
that the blood is condenfed in the lungs ; and others, that
the globules, and all the finer humours, receive their con-
figuration there, ibid. Some, without confidering the

air

Refpiration, and the UJe of the Blood. ?

air as of any other ufe than to put the lungs in motion,
think, that heat is produced in the lungs by the attrition,
of the blood in paffing through them .Mifc. Taurin. voL
V. p. 36. The red colour of the blood has been thought
by fome to be caufed by this attrition in the lungs; but
lower refuted this notion, chiefly by obferving, that the
attrition of the blood is greater in the mufcles, from
which, however, it always returns black, Ibid. vol. I. p. 7 4-
Dr. why tt thought there was fomething of a vital,
and ftimulating nature derived from the air into the
blood, by means of which it made the heart to contract,

HALLER, VOl. III. p. 336.

boerhaave fays, that air not changed is deadly; not
on account of heat, rarefaction, or denfity, but for fome
other occult caufe. Mifc. Taurin. vol. V. p. 30.

Dr. hales, who has thrown much more light upon
the dodtrine of air than all his predeceflbrs, was equally
ignorant of the ufe of it in refpiration; and at different
times feems to have adopted different opinions concern-
ing it.

In his Statical Effays, vol. II. p. 3 2 1 . he fuppofes, that
air is rendered alcaline by breathing, and corrected, in
fome meafure, by the fumes of vinegar.

In agreement, as he obferves, with boerhaave, he fays,
p. 100. that the blood acquires its warmth chiefly in the
lungs, where it moves with much greater rapidity than in,
any other capillary vefTels of the body, vol. II. p. 87;
hut that one rife of the air is to cool the blood, p. 94 ; and
he makes an eftimate of the degree of this refrigeration.
The red colour of the globules of blood, he fays, p. 88,

intimates

8 T)r. priestley's Obfervations on

intimates their abounding with fulphur, which makes
them more fufceptible and retentive of heat than thofe
bodies which have lefs of it.

He alfo fuppofes, p. 102, that another great ufe of the
lungs is to attenuate and feparate the globules of blood;
and that the floridnefs of the arterial blood above the ve-
nal may, in a good meafure, be owing to the ftrong agita-
tion, fri6tion, and comminution, which it undergoes in
palling through them. In like manner, in an experiment
which he made for the purpofe, blood much agitated in a
clofe glafs veffel was obferved to be very florid, not only
on its iurface, but through its whole fubftance, as arterial
blood is, vol. II. p. 1 o 2. I would obferve, however, that
in this expement, the blood muft have acquired its florid
colour from the air with which it was agitated*

He adds, that it is probable, that the blood may, in the
lungs, receive fome other important influence from the
air, which is in fuch great quantities infpired into them.
In other places, however, he explodes the dodlrine of a
vivifying fpirit in the air. It has long, he fays, been the
fubjeft of inquiry to many, to find of what ufe it is in
refpiration ; which, though it may in fome refpe<5ts be
known, yet it muft be confeffed, that wTe are ftill much
in the dark about it, vol. II. p. 102.

Suffocation, he fays, vol. II. p. 27 1 > confifts chiefly in
the falling flat of the lungs, occafioned by the groflhefs
of the particles of a thick noxious air, they being, in that
floating ftate, moft eafily attracted by each other, as we find
that fulphur, and the elaftic repelling particles of air are ;

and

Refpiration, and the Ufe of the Blood. 9

and confequently unelaftic, fulphureous, faline, and other
floating particles, will moft eafily coalefce, whereby they
are rendered too grofs to enter the minute veficles, which
are alio much contracted, as well by the lofs of the elafti-
city of the confined air, as by the contraction occafionedby
the Simulating acid fulphureous vapours. And hence it
is not improbable, that one great defign of nature in the
ftruCture of this important and wonderful vifcus^ was to
frame the veficles fo very minute} thereby effectually to
hinder the ingrefs of grofs, feculent particles, which
might be injurious to the animal economy.

Laftly, he concludes, that the effeCt of refpiration is to
abate, and in part deftroy, the elafticity of the air; and as
this was effected by fuphureous vapours, and he could
breathe for a longer time air that had pafled through
cloaths dipped in a folution of fait of tartar, he con-
cluded, that the air had been mended by the tartar hav-
ing ftrongly imbibed the fulphureous, acid, and wratery
vapours, voL I. p. 267.

haller, after reciting the opinions of all that had
gone before him, fuppofes, with Dr. hales, that, in con-
lequence of the air lofing its fpring in the lungs, they
cannot be kept dilated; and therefore, they muftcollapfe,
and the circulation of the blood be impeded, vol. III. p.
258. When he ftates his opinion concerning the ufe of
the lungs more fully, he fays, that the true ufe of
them is partly inhaling, and partly exhaling, p. 351.
That the lungs inhale both water and air; but that in the
lungs the air lofes its elaftic property, lb as to be eafily
foluble in water or vapour, p. 352,: and he thinks it

B probable,

io Dr. priestley's Obfervations on

probable, that this air ferves as a cement to bind the
earthy parts together. He alfo makes no doubt, but that
various other matters, mifcible with water, are inhaled
by the lungs ; and he even thinks it not improbable, that
the air may carry fome eleitric virtue along with it. The
principal exhalation of the lungs, he thinks, to be water,
abounding with oily, volatile, and faline principles ; and
thefe oily and fetid vapours, he thinks, to be the fuligines
of galen and other ancients, p. 354.

Mr. cigna of Turin, has given much attention to this
curious fubjeft, as appears by two Memoirs of his; one
in the firft volume of the Mifcellanea 'Taurinenfia, in which
he very well accounts for the florid red colour of the
blood; and the other, which is a much more elaborate
Memoir, intitled, DeRefpiratione> in the fifth volume of the
fame work, juft publifhed, or about to be publifhed, the
copy of the article having been fent to me by the author.

He takes it for granted, that air which has once been
breathed is unfit for farther refpiration, on no other ac-
count than its being loaded with noxious vapours, which
difcover themfelves by a fetid fmell. Mifc. Tattrin. vol.
V. p. 30. And he takes it for granted, that the elafticity
of air is diminiihed by refpiration, though he does not
confider that diminution of elafticity as the caufe of its
noxious quality. He therefore concludes, that air which
has been breathed, fuffocates by means of the irritation
which it occafions to the lungs, by which the bronchia,
and the lungs themfelves, are contracted, fo as to refift the
entrance of the air* and therefore, that refpired air is,

noxious

Refpiration, and the Ufe of the Blood. 1 1

noxious on the fame account as mephitic vapours, or thofe
of burning brimftone, p. 3 1 ; that, in frequently breath-
ing the fame air, it becomes fo loaded with thefe vapours,
as to excite a convulfion in the lungs, and thereby render
them unfit for tranfmitting the blood, p. 42.

This philofopher fuppofes that air enters the pores of
the blood, retaining its el aftic power, p. 50. and that it
continues at reft there, becaufe its endeavour to efcape is
counteracted by the equal preffure of the ambient me-
dium, p. 52. This air, he fuppofes to be introduced into
the blood by the chyle, and never by the way of the
lungs, except when, by fome means or other, the equili-
brium between the air in the blood and the external air
is loft, p. 57. If the external air be rarer than the inter-
nal, the air in the blood, expanding itfelf, will inflate the
animal, and have the fame effeit as air introduced into
the veins.

What we are chiefly indebted to M. cigna for, is his
decifive experiments with refpecSt to the florid colour of
the blood, which he clearly proves to be caufed by the
contacSt of air; though he afterwards feems willing to de-
fert that hypothefis. It was often imagined, that the
reafon why the lower part of a quantity of blood was
black, while the furface was red, was, that the black par-
ticles, being heavier than the reft, fubfided to the bot-
tom; but this opinion our author clearly refutes.. He
found, that when he put a little oil upon a quantity of
blood, it remained black throughout ; but that when he
took away the red part, and expofed to the air the lower

B 2 lamina.

12 Dr. Priestley's Obfervations on

lamina, which were black, they alfo became fucceffively
red, till the whole mafs acquired that colour, Mifc. Tail-
rin. vol. L p. 73. Alio, at the requeft of M. cigna, Fa-
ther beccaria tried what would be the effect of expofing
blood in vacuo; and he found, that in thofe circum-
ftances, it always continued black; but that, by expofing
it again to the air, it became red, p. 68.

M. cigna concludes his firft differtation with obferving,
that it is not eafy to fay how it comes topafs,that the lower
part of a mafs of blood becomes black, whether by the air
which it had imbibed efcaping from it, or by its depofit-
ing fomething faline, neceffary to contribute to its red-
nefs, or by the preffure of the atmofphere ; but he in-
clines to think, that air mixed with blood, and interpofed
between the globules, preferves its rednefs : but that by
concreting it is expelled from it, or becomes fo fixed as
to be incapable of making it red. This opinion, he
thinks, is rendered in fome meafure probable, by the in-
creafed denfity of concreted blood, and by the emiffion
of air from other fluids in a concrefcent ftate, p. 74.

Notwithftanding what he had advanced in his firft
Memoir, yet inthefecond, which was written feveral years
after it, he doubts whether the change of colour in the
blood takes place in the lungs ; but if it does, he inclines
to afcribe this effed: to the evaporation from the blood
in the lungs : and though he always found, that the co-
lour of the blood was changed by the contact of air, yet
when he confidered that evaporation muft, as he thought,
nectffarily attend the contact of air, he imagined, that this

effeft:

Refpiration, and the Ufe of the Blood. 1 3

effeft might equally be attributed to this circumftancc.
gut he acknowledges, that this hypothefis ought not to
be received till it be confirmed by experiments, Mifc.
'Taurin. vol. V. p. 61.

Upon the whole, he concludes, that the principal ufe of
air to the blood, is to preferve the equilibrium with the ex-
ternal air, and to prevent the veffels from being rendered
unfit to tranfmit the blood, on account of the external
preffure; whereas, by means of the air they contain, the
fluids move in their proper veffels as freely as in vacuo,
and the membranes and vifcera alfo eafily Hide over each
other, p. 63. And with refpe6t to the ufe of the lungs,
fince he imagined that air is not introduced into the blood
by means of them, he thinks, that becaufe fuch lungs as
thofe of man are given to the warmer animals only, the
chief ufe of refpiration is exhalation, and confequently
the cooling of the blood, p. 65.

The laft wrriter whom I fhall quote upon this fubje£ty
is the late ingenious Mr. hewson; who fays, in his Ex-
perimental Inquiry into the Properties of Blood, p. g.
" As the colour of the blood is changed by air out of the
" body, it is prefumed, that the air in the lungs is the
" immediate caufe of the fame change in the body."
That this change is really produced in the lungs, he is
perfuaded, he fays, from experiments, in which he dif-
tin6lly faw the blood of a more florid red in the left au-
ricle of the heart than it was in the right ; but how this;
effeft is produced, he fays, is not yet determined.

Since

14 Dr. Priestley's Obfervations on

Since fome of the neutral falts, and particularly
nitre, has a fimilar effedt on the colour of the blood;
fome, fays he, attribute this difference to the nitre ab-
forbed from the air, while in the lungs. But this, he
adds, is a mere hypothefis> for air contains no nitre,
and moil of the neutral falts produce the fame effedt in
fome degree.

Since, however, a folution of nitre does produce this
effect upon blood, inftantly making the very blacked of
it of a beautiful florid red, though this effect is not pe-
culiar to nitre (for a folution of common fait does nearly
the fame thing) I own I am inclined to afcribe this effect
to the air; efpecially fince 1 have proved, as I apprehend,
that atmofpherical air confifts of earth and fpirit of nitre.
Poffibly, therefore, the air we breathe may be fo far de-
compofed, as to communicate fomething of nitre to the
blood, in its pafTage through the lungs.

After this review of the obfervations and opinions of
others on this important queftion in phyfiology, I fhali
proceed to recite my own. It may appear fomething. ex-
traordinary, that among fuch a variety of opinions concern-
ing the ufe of refpiration, the right one fhould never have
been fo much as conje£tured, though unfupported by the
proper proof. But indeed, this animal function, and the
phlogiftic proceffes in chemiftry, efpecially that of the
calcination of metals, which is, perhaps, the moft fimple
of them, are to appearance very different things ; and
therefore, it is the lefs to be wondered, that no perfon

fhould

Refpiration, and the life of the Blood. 1 5

ihould have imagined, they would produce the fame
effect on the air in which they were performed.

That refpiration, however, is, in reality, a true phlogiftic
procefs, cannot, 1 think, admit of a doubt, after its being
found, that the air which has ferved for this purpofe is
left in precifely the fame ftate as that which has been ex-
pofed to any other phlogiftic procefs. And fince all the
blood in the body paffes through the lungs, and, accord-
ing to Mr. hewson's obfervations and others, the re-
markable change between the colour of the venal and
arterial blood takes place there, it can hardly be doubted,
that it is by means of the blood that the air becomes
phlogiiticated in paffing through the lungs; and there-
fore, that one great ufe of the blood muft be to difcharge
the phlogiflon with which the animal fyftem abounds,,
imbibing it in the courfe of its circulation, and imparting
it to the air, with which it is nearly brought into contact ,
in the lungs ; the air thus adting as the great menftruum
for this purpofe.

Though J had no doubt concerning this conclufioat
from my former experiments, I thought fo great a pro-
blem deferved as much ilhiftration as could be given to it ;
and therefore I was willing to try, whether the blood was
of fuch a nature, as to retain any of this power of affedt-
ing air when congealed, and out of the body, that it has
when it is fluid, and in the body ; and the experiments
have fully anfwered my expectations.

Having taken the blood of a flieep, and let it ftand till
it was coagulated, and the ferum was feparated from it
(after which the furface, being expofed to the common

air*

1 6 Dr. Priestley's Obfervations on

air, is well known to aflume a florid red colour, while the
infide is of a much darker red, bordering upon black) I
introduced pieces of the craflamentum, contained in nets
of open gauze, or of wire, fometimes through water, and
ibmetimes through quickiilver, into different kinds of air,
and always found that the blackeft parts affumed a florid
red colour in common air, and more efpecially in de-
phlogifticated air, which is purer and more fit for refpi-
ration than common air (and accordingly the blood al-
ways acquired a more florid colour, and the change was
produced in lefs time in this than in common air) whereas
the brighteft red blood became prefently black in any
kind of air that was unfit for relpiration, as in fixed air,
inflammable air, nitrous air, or phlogifticated air; and
after becoming black in the laft of thefe kinds of air, it
regained its red colour upon being again expofed to com-
mon air, or dephlogifticated air ; the fame pieces becom-
ing alternately black and red, by being transferred from
phlogifticated to dephlogifticated air ; and vice verfa.

In thefe experiments the blood muft have parted with
its phlogifton to the common air, or dephlogifticated air,
and have imbibed it, and have become faturated with it,
when expofed to phlogifticated, nitrous, inflammable, or
fixed air. The only difficulty is with refpecl to the fixed
air; for all the other kinds certainly contain phlogifton.
But, as I have obferved in the account of my experi-
ments on vitriolic acid air, phlogifton feems to be necef-
fary to the conftitution of every kind of air; and befides,
the blacknefs of the blood may arile from other caufes

than

Refpiration, and the Ufe of the Blood. 1 7

than its acquiring phlogifton. gaber, for inftance, ob-
ferves, that blood becomes black when it begins to pu-
trify, as it does alfo whenever it is dried and hardened
near the fire. Father beccari a alfo found, as I have ob-
ferved, that red blood continued (and he could hardly fail
to obferve alio, that it became) black in vacuo, where it
could not have imbibed phlogifton. This I found to be
the cafe when the blood was covered two inches and a
half with ferum ; but it regained its florid colour when
it was expofed to the open air.

A In general, however, it cannot be expelled, that when
blood has become black without having received phlo-
gifton ab extra, it will recover its florid colour by being
expofed to the air. For the delicacy of its texture, and
confequently its capacity of being eafily affeited by phlo-
gifton, may be effentially altered by internal caufes of
blacknefs. This is even the cafe when blood has become
black by being expofed to nitrous and inflammable air,
though this change is probably effedted by its imbibing
phlogifton.

I expofed pieces of the fame mafs of red blood to thefe
two kinds of air, and alfo to fixed air at the fame time.
They all became black ; but that which was in the in-
flammable air was the leaft fo, and none of them reco-
vered their florid colour in the open air. But at another
time, a piece of craflamentum, which had become black
in fixed air, did, in fome meafure, and very flowly, reco-
ver its florid colour in dephlogifticated air. Perhaps
the pieces that had loft their colour in the nitrous and

C inflam*

1 8 Dr. Priestley's Obferv.

inflammable air might have recovered it by means of this
more powerful menftruum.

Since, however, blood, after becoming black in phlo-
gifticated air, is always capable of reluming its red co-
lour on being again expofed to pure air, it may be con-
cluded, that the preceding blacknefs, difcharged in the
pure air, and producing the conftant effeit of phlogifton,
ing the air, was owing to the phlogifton it had
imbibed in the former fituation, and which it parted with
in the latter. And this is remarkably the cafe when
blood is transferred from phlogifticated into dephlogifti-
cated air. Even the circurnftance of the deeper colour is
fufficient to give a chemift a fufpicion that it contains
more phlogifton than blood of a lighter colour.

When I had found how readily pieces of blood changed
their colour, according to the quality of the air to which
they were expofed, I proceeded to examine the ftate of
that air, in order to obferve what change had taken place
in it; and as dephlogifticated air admits of a more fenfi-
ble change of quality than common air, I gave it the pre-
ference in this experiment; putting a piece of crafla-
mentum, about the bignefs of a walnut, into the quantity
of about five ounce meafures of this air.

This procefs I continued for the fpace of twenty-four
hours, changing the blood about ten or twelve times;
after which I found the air fo far depraved, that whereas,
at the beginning of the experiment, one meafure of it and
two of nitrous air occupied the fpace of no more than
half a meafure, the fame mixtures afterwards occupied

the

Refpiration, and the Ufe of the Blood. 1 9

the fpace of a meafure and a half. Now fince air is uni-
verfally depraved by phlogifton, and in this fenfe, I be-
lieve, by nothing elfe, it is evident, that this black blood
mult have communicated phlogifton to the air; and con-
fequently its change of colour from black to a florid red
muflr have been occafioned by the reparation of phlo-
gifton from it.

The next day, when, of courfe, the blood was nearer
to a ftate of putrefadlion, in which every kind of iub-
ftance, without exception, will injure refpir able air, I put
a quantity of red blood, tinged in a few places with black,
which I could not eafily feparate from it, to about the fame
quantity of the fame dephlogifticated air, and fufFered it
to ftand, without changing, for the 'fame fpace of time;
when it was fo little injured, that the meafures abovemen-
tioned occupied the fpace of only two-thirds of a meafure.

That blood has a power of taking phlogifton from
air, as well as imparting phlogifton to air, I fatisfied my-
felf by expoiing blood of a very beautiful florid colour
to nitrous air, inflammable air, and phlogifticated air.
The two firft mentioned kinds of air were coniiderably
diminifhed by the procefs, which was continued two
days, during which time the blood had been changed
five or fix times.

The nitrous air, by this means, loft a great proportion
of its power of diminifhing, that is, phlogifticating,
common air. For now two meafures of common air and
one of this occupied the fpace of 2^, inftead of i| mea-
fures. The inflammable air, though ftill inflammable,

C 2 was

20 Dr. priestley's Obfervations on

was rendered in fome degree wholefome by the procefs ;
being, after this, confiderably diminished by nitrous air,
which is a ftate to which it is brought by agitation in
water, and which, continued longer, deprives it of its in-
flammability likewife. It cannot be doubted, therefore,
but that, in both thefe cafes, the red blood, by becoming
black, received phlogifton from thefe two kinds of air.

ith refpect to the phlogifticated air, I only obferved
that, after a few hours expofure to the red blood, it was
f enfibly, but not much, diminifhed by nitrous air, which
otherwife it would not have been in the leaft degree.
This blood, however, was of the light eft colour; that is,
according to my hypothefis, the moft free from phlo-
gifton, of any that I have ever feen ; and I have tried the
fame thing, without fuccefs, with blood of a lefs florid
colour, though as florid as the common air could make it.
But it fhould be confidered, that the proper function of the
blood is not to receive phlogifton from #/>, not meeting
with any phlogifticated air in the courfe of its circula-
tion, but to communicate phlogifton to air; and there-
fore, there is by no means the fame reafon to expeit, that
air will be mended by red blood, as that it will be injured
by black blood.

It may be objedted to this hypothefis, concerning the
ufe of the blood, that it never comes into actual conta6t
with the air in the lungs, but is feparated from it, though
as Dr. hales ftates it, at the diftance of no more than a
thoufandth part of an inch. The red globules alfo fwim

in

Refpirailon, and the Ufe of the Blood. % i

in a large quantity of ferum, which is a fluid of a quite
different nature.

In order to afcertain the effe£l of thefe circumftances,
I took a large quantity of black blood, and put it into a
.bladder moiftened with a little ferum, and tying it very
clofe, hung it in a free expofure to the air, though in a
quiefcent itate ; and the next day I found, upon exami-
nation, that all the lower furface of the blood, which had
been feparated from the common air by the intervention
of the bladder (which is an animal membrane, fimilar to
that which conftitutes the veficles of the lungs, and is at
leaft as thick) and likewife a little ferum, had acquired a
coating of a florid red colour, and as thick, I believe, as
it would have acquired, if it had been immediately ex-
pofed to the open air ; fo that this membrane had been no
impediment to the adtion of the air upon the blood. In
this cafe it is evident to obferve, that the change of co-
lour could not be owing to evaporation, as Mr. cign a con-
jectures. This experiment I repeated, without previoufly
moiftening the bladder, and with the very fame refult.

I obferved alfo, that when I cut out a piece of the
craffamentum, and left the remainder in the veffel
writh the ferum, not only that part of the furface which
was expofed to the air, but that which was furrounded
with ferum, and even covered with it to the depth of fe-
veral inches, acquired the florid colour; fo that this deep
covering of ferum, which muft have effectually prevented
all evaporation, was no more an impediment to the mu-
tual action of the blood and the air, than the bladder had

been.

22 Dr. priestley's Obfervations on

been. The ferum of the blood, therefore, appears to be
as wonderfully adapted to anfwer its purpofe, of a vehicle
for the red globules, as the red globules themfelves : for
the flighted covering of water, or faliva, effectually pre-
vents the blood from acquiring its florid colour; and Mr.
cigna found that this was the cafe when it was covered
with oil.

That it is really the air, a6ting through the ferum, and
not the ferum itfelf, that gives the florid colour to the blood,
is clearly afcertained by the following experiment. I
took two equal portions of black blood, and put them
into equal cups, containing equal quantities of ferum,
which covered them to the depth of half an inch. One
of thefe cups Handing in the open air, and the other being
placed under an exhaufted receiver, the former prefently
acquired a florid colour, while the other continued twelve
hours as black as at firft. Being taken out of the receiver,
it flood all night in the open air without becoming red,
and continued black ever after, even when the ferum was
poured off.

I alio more completely fatisfied myfelf of the influence
of the air upon the blood, through a body of ferum, by
the reverfe of this experiment. For I found that red
blocd became black through the depth of two inches of
ferum, when the veflel containing it was expofed to
phlogifticated air; fo that the red globules of the blood
both receive, and part with phlogifton by means of the
air, notwithftanding the interpofition of a large body of
the fluid in which they naturally float.

Except

Refpiratmiy and the Ufe of the Blood. 1 3

Except ferum, milk is the only animal fluid that I have
tried/ through which the air can act upon blood: for
black blood became red when it was plunged in milk, in
the fame manner as if it had been covered with ferum. In
urine, indeed, black blood becomes inftantly red; but
this is not owing to the adtion of the air, through the
urine, but to the faline nature of that fluid.

In fome cafes, care rauft be taken to diftinguifh the
floridnefs with which fome detached parts of a quantity
of blood are tinged, from that which penetrates the folid
parts of it. In faliva^ and in water impregnated with
alkaline fait, fixed or volatile, and alfo in fpirit of wine,
the extreme angles and edges of pieces of craflamentum
and fmall detached parts, floating in thofe liquors, will
appear of a very florid red, while the compact mafs of
blood continues dark. The florid colour of the promi-
nent and detached parts, in thefe cafes, feems to be the
mere effect of the minute diviiion of the parts of the
craflamentum in the fluid in which thofe parts float;
when at the fame time it has no fuch effedt on thofe
parts which remain compact, nor has the air the leaft
-power of adting on the blood through the liquor.

I had imagined, that iince black blood contains more
phlogifton than red blood, that difference would have
appeared in the air produced from them, either by being
Amply diflblved in fpirit of nitre, or when dried and
made into a pafte with that acid. But the difference wras
too fmall to be fenfible to this kind of telt. For this
purpofe, however, I had fome blood drawn from the vein

of

1 4 Dr. Priestley's Observations on

of a fheep, and alfo took fome that came firft after killing
it, as the butchers ufually do, by dividing the carotid ar-
tery ; but though I diffolved the black part of the for-
mer, and the red part of the latter, in equal -quantities of
the fame fpirit of nitre, I found ho fenfible difference in
the air that they yielded. The air that I got from them
when dried, and made into a pafte with fpirit of nitre,
was likewife equally indiftinguifhable. The quantity of
air from this procefs was very great, and was produced
irregularly, as I have obferved it to have been when pro-
duced by a folution in fpirit of nitre without drying.
Obfervations on Airy vol. II. p. 155. Half of this pro-
duce was fixed air, and the reft phlogifticated, except that
a candle burned in it with a lambent blue flame. It is
evident, however, from this experiment, that even the
moft florid blood contains a confiderable quantity of
phlogifton; for, other wife, this air would have been de-
phlogifticated*

I would conclude this paper with obferving, that I
have found a very great difference in the conftitution of
blood with refpedt to its property of being affeited by the
influence of the air; fome becoming very foon of a light
florid colour, and the Jlratum of this colour foon grow-
ing very thick; whereas, in other cafes," the colour of
the blood, in the moft favourable circumftances, has con-
tinued much darker, and the lighter colour has never
penetrated very f ar/ ^^^

As the principal ufe of the blood feems to be its power

<of receiving and difcharging phlogifton, and the degree
jNDED 1 Wrz .

cheading:3 Respiration . and the Use of the Blood.

in which it possesses this power is easily ascertained by
the eye, it might not, perhaps, be unworthy of being par-
ticularly attended to by physicians. To estimate the
goodness of blood, according to this criterion, nothing is
requisite but to observe the lightness of the colour, and
the depth of the 1 Irjht- coloured stratum, after it Las been
exposed to the air for a given time. In cases in which
the blood is unusually black, and but little affected by
common air, it should seem, that breathing a purer air
might be prescribed with advantage.

In general, the Blood that I have been able to procure
in the city has not been so good as that which I have got
in the country; owing, perhaps, to the cattle having been
much driven, and heated before they were killed .

cEnds here in middle of p. 25. Transcribed, 1941, from
the Osier Library copy of Priestley's "Observations on
Respiration ...", 1776. 3