PHYSIOLOGY. 
35S 
reptiles, fifhes, and moll of the lower claffes, is (low and 
languid ; and the temperature of thefe animals is pro¬ 
portionally low. The heat of each fpecies is, however, 
pretty uniform under ordinary circumftances. That of 
the human body is generally about 98° of Fahrenheit. 
This however depends on the circumllances in which it 
is placed. When much chilled by the aftion of cold, the 
temperature of the human body falls a degree or two 
below the ordinary height; and under the influence of 
violent fever, it rifes feveral degrees above it. The tem¬ 
perature is generally highelt in children; and inftances 
are recorded of thefe having furvived, while their mothers, 
to whofe breads they clung, have periflied from the 
feverity of the cold. 
One of the mod intereding fadls relating to the fubjefl 
of animal heat, is the capacity of preferving the equable 
temperature of their bodies, poflefled by mod animals. 
Man himfelf can live with little inconvenience in the 
frozen regions of Spitzbergen, and under the equatorial 
heats of Africa. He can even fupport a greater degree 
of heat than is perhaps ever known to take place from 
the rays of the fun, as is proved by the experiment of 
Drs. Blagden and Fordyce in heated rooms ; thefe gen¬ 
tlemen having remained for 15 minutes together in a heat 
exceeding 130 0 . and the heat fupported by fome of the 
inferior animals is dill more extraordinary. 
No explanation of the mode in which the air contri¬ 
butes to fudain animal heat was attempted, till after the 
great difcovery of latent heat by Dr. Black. That ex¬ 
cellent philofopher, having already proved that the change 
efrefted in the air by refpiration confided in the formation 
of carbonic acid, iimilar to what happens in many ex¬ 
amples of combudion, afcribed the production of animal 
heat to the decompofition of the air in the lungs, by 
which its latent heat was rendered fenfible, in the fame 
manner as it is given out in combudion. The blood, in 
its pafl'age through the lungs, had, he conceived, its tem¬ 
perature by this means railed ; and thus was rendered 
capable of communicating heat to all parts of the body, 
in the courfe of its circulation through the fydem. To 
this it was objeCted by Dr. Cullen, that, if true, the tem¬ 
perature of the body ought to be greated in the lungs, 
and to diminifli gradually as the didance from the lungs 
increafes, which is not according to fail. This difficulty 
was removed by the ingenuity of Dr. Crawford, who, 
maintained, that the heat, liberated by the decompofition 
of the air in the lungs, palled into the blood, and exifted 
in that fluid in the form of latent, or what is now termed 
fpecific heat, in confequence of which its temperature was 
not railed ; and that this heat, by other chemical changes, 
was given out by the blood in a fenfible form during its 
circulation. 
In what manner, then, does the air, breathed by the 
fuperior animals, give out its hear, to fupport that high 
degree of temperature above the furrounding medium, 
■which they all poflefs ? We have feen reafon to conclude, 
that the infpired air is decompofed in the bronchial cells 
of the lungs, and that all its oxygenous portion which 
dii'appears is converted into carbonic acid, by carbon 
emitted from the exhaient furface of thofe organs. 
During this gradual converfion of the oxygen gas, a 
quantity of fpecific caloric, much greater than what is 
neceffary to maintain the elafticity of the carbonic acid 
that is formed, is necefl’arily fet free; and to this excels 
of heat, thus conflantly liberated in the lungs by the de¬ 
compofition of the air, do we look as the fource of that 
fuperiority of temperature, above the furrounding 
medium, which man and other animals, under every 
vicifiitude of climate, are enabled to exhibit and main¬ 
tain. 
But, if a quantity of caloric be thus conflantly dis¬ 
engaged in the lungs, it may be expeCted that the blood, 
in its tranfmiffion through thofe organs, fhould acquire a 
certain portion of it. To afcertain this point, Dr. 
Crawford, purfuing the difcoveries of Drs. Black and 
Irvine, mixed together certain quantities of water, at the 
temperature of 53 0 , with feparate portions of arterial and 
venal blood ; and then, meafuring the heat of the mixture, 
at different fucceflive periods, till coagulation took place, 
he found that the water containing arterial blood pre- 
ferved a fuperiority of temperature over that mixed with 
venal blood ; and, from the refults of feveral trials, he 
concluded, that the fpecific heat of the arterial blood of 
a dog was to that of the venal as 114 to 100, and that 
of a fheep as 115 to 100, or as nftoio. (On Animal 
Heat, p. 279.) Thefe refults derive confirmation from the 
experiments of Mr. Coleman, who, in order to difcover 
the relative fpecific heat of arterial and venal blood while 
yet retained in the fyftem, ftrangled a cat, and imme¬ 
diately opened its cheft, while the blood in the left ven¬ 
tricle was flill florid. He then introduced a thermometer 
through an opening in the pericardium on each fide of 
the heart, and it flood at 98°: in the left ventricle the 
temperature was only 97 0 , and in the right ventricle it 
was nearly 99 0 . In fifteen minutes, however, inllead of 
the right ventricle pofleffing two degrees of heat more 
than the left, it was found to have four degrees lefs. 
Mr. Afiley Cooper repeated this experiment in different 
ways; and found invariably, that, although the venal blood 
was fuperior in temperature at firft, yet, before coagulation 
was complete, the arterial beer.me from three to fix degrees 
warmer. (On Sufpended Refpiration, p. 4.2, et feq.) Thefe 
fa£ts are, however, directly contradicted by a very ac¬ 
curate experimentalift, Dr. Davy, in the Phil.Tranf.1814. 
The following is an analyfis of the opinions of 
Crawford, whofe account of animal heat is commonly 
called the chemical theory. This author firft makes 
thofe obfervations on heat in general. 1. That heat has 
a tendency to diffufe itfelf. 2. That the quantity of heat 
in a body is limited. 3. That the quantity of heat in a 
homogeneous body of a given temperature is as its bulk. 
4. That the variations in the quantity of abfolute heat 
in a homogeneous body, while it retains its form, are 
nearly proportional to its variations of temperature, as 
meafured by a mercurial thermometer. 5. That the ca¬ 
pacities of bodies for containing heat are nearly per¬ 
manent, while they retain their form. 6. That when, by 
expofure to heat, folid bodies become fluid, they abforb 
a portion of heat which is neceffary to their fluidity, and 
which does not increafe their temperature; and when, 
by expofure to cold, they become folid again, they part 
with the heat which they had before abforbed ; that, 
when water is converted into vapour, a quantity of heat 
is abforbed, which does not increafe the temperature. 
And, 7. That unequal quantities of abfolute heat are 
neceffary, to produce equal changes of temperature in 
heterogeneous bodies. 
Though feveral of thefe faCts are fufficiently evident 
to be admitted as axioms, yet others are not fo, and even 
fome queftion flill exifts as to their accuracy ; but, fup- 
pofing. that doubtful, the circumftance will fcarcely 
affeCt the general conclufions refpeCting the origin and 
diffufion of animal heat. Crawford advances the following 
feries of propofitions : 
Firft, “ That the quantity of abfolute heat contained 
in pure air (oxygenous gas) is diminifhed by the change 
which it undergoes in the lungs of animals; and the 
quantity of heat in any kind of air that is fit for refpi¬ 
ration, is nearly proportional to its power of fupporting 
animal life.” This propofition is fupported by the refults 
of a great variety of very ingenious but delicate expe¬ 
riments on the heat imparted by elaftic fluids. The 
former part of the propofition is the only one eflential to 
the theory; the latter part is probably incorreCt, as 
neither the nature nor number of elaftic fluids was well 
known at that time. 
His fecond propofition is, “ that the blood which pafles 
from the lungs to the heart, by the pulmonary vein, 
contains more abfolute heat than that which paffes from 
the heart to the lungs by the pulmonary artery.” To 
eftablifh 
