PHYSIOLOGY. 
cup. Tire internal furface of the jar was foon bedewed 
with moifture, and a pellicle began to form on the lime- 
water, which, in a few hours, was iticreafed to a thick 
cruft of carbonate of lime. The craffamentum was then 
removed, and a frefh giafs of lime-water was placed in 
the ferum, which, in thirty-fix hours, had acquired a 
cruft like the former, and the water had rifen confidera- 
bly into the jar. In another experiment, where the fe¬ 
rum was placed for twenty-four hours in ajar of air in¬ 
verted in mercury, the refidual air rendered lime-water 
milky, and the remainder had loft a part of its oxygen. 
A fimilar production of carbonic acid feems to have oc¬ 
curred, when, with a fmall diminution of the gas, a (light 
change of colour was produced on venal blood by placing 
it in contaCt with nitrous oxyd, in the experiments of 
Mr. Davy ; for, when a folution of ftrontian was admitted 
to the oxyd, it became (lightly clouded, and, with the 
diminution of bulk that followed, minute portions of 
carbonic acid and nitrogen gits were produced. (Re- 
fearches, p. 377, 380, 387.) 
Hence then we learn, that, when venal blood is expofed 
to the contaCt of atmofpheric air, of oxygen gas, or of 
nitrous oxyd, it prefently aflumes a florid colour, and, at 
the fame time, the volume of air is fomewhat dimini(hed) 
and a portion of carbonic acid is produced. Does then 
the carbonic acid, which is here met with, proceed ready 
formed from the blood, or is it in part formed by the 
decompofition of the air ? No one has yet proved that 
any aeriform fluid, much lefs that carbonic acid, exifts 
naturally in the blood; and if this be true, no fuch 
aerial acid can be expeCted to iffue from it. The carbonic 
acid alfo, is not formed by blood when it is confined in 
nitrogen gas ; neither does the colour of the blood, in 
that cafe, undergo any fenfible change; but this acid is 
formed by blood, either in oxygen gas, in nitrous oxyd, 
or in atmofpheric air, all of which are deteriorated 
thereby ; whence it follows, that, without the prefence of 
oxygen gas, the blood is unable to form carbonic acid ; 
and that this acid, therefore, is in part formed out of the 
gas. If the oxygen gas that difappears does not contri¬ 
bute to form the carbonic acid that is produced, in what 
other manner can its lofs be accounted for ? or from what 
other fource than the oxygen gas of the air, in contaift 
with the blood, can that ingredient of the acid be derived ? 
Thofe who fuppofe the carbonic acid to be furniflied by 
the blood, independent of the air employed, muft likewife 
fuppofe that the nitrogen gas is furniflied by it alfo; for 
the experiments of Mr. Davy teach 11s, that a portion of 
that gas, as well as of carbonic acid, is always prefent 
when nitrous oxyd is decompofed, which renders it pro¬ 
bable that the fame thing likewife occurs when air is 
changed by venal blood. But in what manner the blood 
fliould be able to furniflt nitrogen gas, it is not eafy to 
conceive, fir.ce no affinity exifts between that gas and 
venal blood. (Davy’s Refearches, p. 375.) We infer, 
therefore, from thefe fads, that atmofpheric air is decom¬ 
pofed by being placed in contad with venal blood, its 
oxygenous portion being in part converted into carbonic 
acid, and a quantity of its nitrogen being, in confequence, 
left free. 
But, fuppofing the air to be thus decompofed by the 
blood, it ftiil remains a queftion, whether it has been firft 
attraded by that fluid, then decompofed, and afterwards 
in part expelled; or, whether the decompofition has been 
effeded without fuch previous attradion and intermix¬ 
ture of air. The only evidence of this fuppofed attradion 
(eems to be the fmall diminution of bulk which the air in 
all cafes fuffers : but this cannot be confidered as a proof 
of the attradion of the air; for it is a necefl'ary confe¬ 
quence of that converfion of oxygen gas into carbonic 
acid, which has been (hown to take place when thefe fub- 
ftances are brought into contad. Even granting to the 
blood this power of attrading air, or its oxygenous por¬ 
tion, it is not eafy to conceive why it fliould fo readily 
lofe it, and again give out this air in the form of carbonic 
35a 
acid. No change of quality in the blood, nor any varia¬ 
tion of temperature, can have taken place fufficient to 
alter fo rapidly its affinity for thefe fubftances : and it 
cannot proceed from a want of affinity between the blood 
and the carbonic acid that is formed ; for that acid fuffers 
a greater diminution, either than oxygen gas or atmof¬ 
pheric air, by being placed in contad with blood. We 
incline, therefore, to the opinion, that neither the air nor 
its oxygen gas is attraded by and diffufed through the 
blood, as happens with feveral gafes when placed in 
contad with certain fluids : but that the air is decom¬ 
pofed, and its oxygen gas changed into carbonic acid, 
without entering into the fubftance of that fluid. 
But, for the formation of this acid, the blood muft 
fupply carbon, fince no other fubftance was prefent from 
which it could be derived; and it is well known alfo, that 
carbon enters largely into the compofition of that fluid; 
and our experiments prove, that it exifts as well in the 
ferous as in the more folid parts. By fome it may be ob- 
jeded, that, becaufe carbonic acid is formed diredly by 
the combuftion of charcoal, it cannot be produced at fo 
low a temperature as exifts in thefe experiments. To 
this we can reply only by an appeal to the general fads 
exhibited through the whole courfe of our enquiry, by 
which it appears, that, both by the living fundions of 
vegetables and animals, and by the decompofition of ani¬ 
mal and vegetable matter, this acid is i» like manner 
formed at temperatures equally low. Even thofe, who 
confider this acid to have proceeded ready formed from 
the blood, cannot attribute its produdion to the opera¬ 
tion of heat: for in the animal body the temperature 
of the blood feldom exceeds ioo°, a degree of heat in¬ 
competent to form carbonic acid by any procefs analo¬ 
gous to combuftion. The combination of many bodies 
is, indeed, greatly accelerated by being expofed to very 
high temperatures ; but this furely does not fet afide the 
fad of their fpontaneous union at temperatures much 
more low. From this review of the effeds which take 
place between the blood and air, we conclude, that the 
chemical phenomena, which arife when the fubftances 
are placed in contad, do not prove an attradion and dif- 
fufion of air through the blood ; but fliow only that a 
reciprocal adion takes place, by which a new produd is 
formed : no inference, therefore, in favour of an attradion 
of air by the blood in the lungs, can be drawn from the 
reciprocal adion which they exert on each other out of 
the body. The fecretion from the membrane of the 
lungs, becoming evaporated on the contad of air, is 
emitted from the lungs in the form of vapours in various 
proportions. 
4. The theories of refpiration, founded on the pre¬ 
ceding fads, are many. Dr. Prieftley confidered ref¬ 
piration to be owing to the difengagement of phlogifton 
from the blood in the lungs, and its combination with 
the air, (Phil. Trans. 1776.) a theory modified and ren¬ 
dered more comprehenfive by Crawford ; (Experiments 
and Obfervationson Animal Heat.) Regarding hydrogen 
as thd*phlogiftic principle, and fuppofing it to exift in the 
blood in that ftate in which it is difengaged from vegetable 
fubftances by heat, the heavy inflammable air of the older 
chemifts, the carburetted hydrogen of the modern no¬ 
menclature. he fuppofed that this hydro-carbon, as it was 
named, is communicated to the blood in the extreme 
veffels, by which the converfion from the arterial to the 
venous ftate is occafioned ; in the lungs, he concluded, it 
is given out, and in its nafeent ftate, or its tranfition to 
the elaftic form, it combines, he fuppofed, with the oxygen 
of the air, and forms the carbonic acid gas and watery 
vapour expired, while the blood, deprived of its hydro¬ 
carbon, returns to the arterial ftate. Nearly the fame ex¬ 
planation was given by Lavoifier ; at lead: he advanced 
the opinion, that the carbonic acid gas and watery vapour 
of the expired air are formed by the combination of carbon 
and hydrogen from the blood with oxygen in the lungs. 
Lavoifier had alfo fuggefted, that the combination of 
oxygen 
