-350 
PHYSIOLOGY, 
different individuals; from the relative vigour or debility 
of the mufcular powers carrying on the reformatory func¬ 
tion ; from the circutnftances in which the animal is 
placed ; the compofition of the air itfelf; or the manner 
in which it is breathed. In many modes of’experiment 
alfo, the friction between the air and apparatus employed, 
or the refiftance which this latter may create to the ordi¬ 
nary procefs, will greatly vary the refult: and confider- 
able errors muff likewife have arifen from the variation 
in bulk, occafioned by the change of temperature, which 
the air, during its refpiration, buffers ; from the difficulty 
of breathing in a natural manner when the mind is di¬ 
recting that procefs; and from the embarraffments op- 
pofed to the natural adfion of the refpiratory organs by 
the contrivances adapted to them. It is not to be fuppo- 
fed that the lungs are all entirely emptied ; even in death 
this does not take place. In life it may be eftimated, that 
by each ordinary expiration one-feventh part of the whole 
contents of the lungs is difcharged, and that by the molt 
violent expiration fomewhat more than four-fevenths of 
the air contained in them is evacuated. Suppofing that 
each refpiration occupies about three feconds, a bulk of 
air nearly equal to three times the whole contents of the 
lungs will be expelled in a minute, or about 4114 times 
their bulk in 24 hours. The quantity of air refpired 
during the diurnal period will be 1,152,000 cubic inches, 
or 666£ cubic feet. 
2. With refpeCt to the changes which refpired air un¬ 
dergoes, it may be remarked, that the ffrff accurate no¬ 
tions refpeCting this change were furniflied by the experi¬ 
ments of Boyle. He not only proved, by means of the 
air-pump, the abfolute necefiity of air to the fupport of 
animal life, but he farther difcovered, that the action of 
the lungs is quickly fufpended, unlefs they are furnilhed 
with a regular fupply of freffi air. He thought, however, 
that the air, in palling through the lungs, acquired, what 
he calls, recrementitious ffeams, an idea fuggefted by the 
quantity of aqueous vapour which he found return 
from thofe organs. Heobferved alfo, that the air in which 
an animal had refpired for fome time was confiderably 
diminiflied in volume, an effedf which he attributed to 
thelofs of part of its elafticity or fpring. 
The contemporaries of Boyle, for the mod part, 
coincided with him in his ideas refpe&ing refpiration ; 
there were, however, fome p'nilofophers, who fuppofed, 
that belides the addition of thefe vapours, the air, during 
its continuance in the lungs, imparted l’omething to the 
blood. Among thefe, the ffrff in point of genius and 
originality was Mayow of Oxford, He inveftigated the 
properties of the air, and the effefts produced upon it by 
refpiration, with great acutenefs, and concluded, that a 
peculiar volatile Ipirit, which was one of the conffituents 
of the atmofphere, was abforhed by the blood during its 
pafiage through the lungs. Borelli, Lower, Willis, and 
others, adopted opinions in many refpedfs ffmilar to that of 
Mayow; they imagined, that either a portion of the whole 
mafs of air, or fome particular conffituent of it, was ab- 
forbed by the blood, and by this means converted this 
fluid from the venous to the arterial ftate. But they en¬ 
tirely failed in their attempts to afcertain the nature of 
the matter ablorbed. 
Dr. Hales devoted much of his attention to this fubjedf, 
and performed many experiments with a view to illullrate 
the manner in which the air is affefted by the lungs; he 
concludes nearly as Boyle had done, that it acquires a 
noxious vapour, and that its elafficity is diminiflied. 
(Statical Eflays.) The learned Boerhaave confeffes his 
inability to explain the changes which the air experiences 
by refpiration. (Praeledf. tom. v. p. 169, et feq.) The 
opinion of Haller was not materially different from that 
of Boyle and Hales; he had colledfed all the different 
theories which have been advanced upon this fubjedt; and, 
after reviewing them with his accuftomed candour and 
perfpicuity, he concludes, that the air, when it is emitted 
from the pulmonary veficles, is combined with a quantity 
of water, and a peculiar noxious vapour, and has its 
elafticity diminiflied. (Notae ad Boerhaav. Praeledl. t. v. 
p. 170. Element. Phyfiol.) 
Dr. Black, by his experiments upon fixed air, was the 
firft who (bowed that the peculiar gafeous fubftance gene¬ 
rated in the lungs during refpiration is fixed air. “So 
early as theyear 1757,” fays this philolopher, “I convinced 
myfelf, that the change produced in wholefome air by 
breathing it, confifts chiefly, if not folely, in the conver- 
fion of part of it into fixed air; for I found, that, by 
blowing through a pipe into lime-water, or a folution of 
cauftic alkali, the lime was precipitated, and the alkali 
rendered mild.” (Black’s Lectures, by Robinfon, vol. 
ii. p. 87.) At a later period, Mr. Bewle^ deteited the 
formation of carbonic acid in refpiration by a method 
fomewhat fimilar: he found, that on breathing through 
an infufion of litmus, the fame change to a red colour 
was produced in it, as when it was expofed to the adtion 
of fixed air; and when, by adding a few drops of the 
water of potaffa, the blue colour was reftored to the infu¬ 
fion, it could again be made to difappear by fuperfatura- 
ting it with the acid expired from the lungs. (Prieftley on 
Air, v. 383.) 
The particular fubftance which conftituted the whole¬ 
fome part of the atmofpheric air, was not, however, 
known to Dr. Black at the time his experiments were 
made: and, long before the compound nature of the 
atmofphere was alcertained, it had been fuppofed by many 
philofophers, that, to ufe the language of biftiop Berkeley, 
“ there W’as no fuch thing as a pure fimple-element of air. 
There is,” he adds, “ fome one quality or ingredient in 
the air on which life more immediately and principally 
depends. What that is, though men are not agreed, yet 
it is agreed it muff be the fame thing that fupports the 
vital and the common flame ; it being found, that when 
air, by often breathing in it, is become unfit for the one, 
it will no longer ferve the other. This quality of the air 
is neceflary both to vegetables and animals, whether 
terreftial or aquatic ; neither beads, infedts, birds, nor 
fifties, being able to fubfilt without air; and, when air is 
deprived of this ingredient, it becometh unfit to maintain 
either life or flame, even though it (hould retain its elafti¬ 
city.” (Siris, § 143, et feq. 2d edition.) Dr. Hooke 
afferted, that this ingredient or fubftance, inherent in, 
and mixed with the air, is like, if not the very fame, with 
that which is fixed in faltpetre, by which, during corn- 
bullion, inflammable bodies are difl'olved. (Micrographia, 
p. 103.) The fame opinion was afterwards held by 
Willis, Lower, and Mayow, all of whom likewile conli- 
dered the nitrous quality of the air to adl an important 
part in refpiration. The laft author, in particular, made 
experiments precifely fimilar to thofe which have lately 
been brought forward to prove, that both by the burning 
of a candle and other combuftible bodies, and by the 
refpiration of animals, the nitro-aerial particles of the air 
were exhaufted, whereby the volume of air was diminifhed, 
and the refidual air was unable afterwards to fupport 
either life or flame. (Tracfat. Quinque, p. 98, et feq-) The 
exhibition, however, of this peculiar, or nitro-aerial, part 
of the air in a diliindt and feparate form, we owe to the 
genius of Scheeleand Dr. Prieftley, who difcovered, inde¬ 
pendently of each other, in the year 1774, pure or dephlo- 
giflicated air, or oxygen gas. The atmofphere, which, 
until this period, had been regarded as a homogeneous 
elementary body, was difcovered by thefe celebrated 
experimenters, to be compounded of two aeriform fluids, 
poffefling diftindl properties, and having totally different 
purpofes in the economy of nature. Thefe fubftances, 
which have fince obtained the names of oxygenous and 
azotic gafes, were found to dxift in the atmofphere, in the 
conftant proportion of about 22 to 78. This great difco- 
very, and the ufe which he made of it, enabled Dr. 
Prieftley to propofe the firft confident explanation of the 
phenomena of refpiration that had ever been offered to 
the public 5 and, although the theoretical opinions on 
, which 
