PHYSIOLOGY. 
eftablifli this, experiments were made on mixtures of 
given weights of water and arterial blood, and water and 
venous blood ; from which it appeared that more heat 
was imparted by arterial blood to water than by venous 
blood, all other circumftances being the fame. 
The third propofition is, s( that the comparative quan¬ 
tities of heat in bodies fuppofed to contain phlogifton 
(combultible) are increafed by the changes which they 
undergo in the proceffes of calcination and combuftion.” 
This propofition, relating more immediately to the theory 
of combuftion than to that of animal heat, needs no ani- 
madverfion here. But, though he confirms the faft by 
various experiments, as far as regards the particular 
bodies operated upon, it does not feem probable that the 
propofition is univerfally true. 
The fourth propofition is, “ that when an animal is 
placed in a warm medium, the colour of the venous blood 
approaches more nearly to that of the arterial than when 
it is placed in a cold medium ; the quantity of refpirable 
air which it phlogifticates (that is, oxygen which it con¬ 
verts into carbonic acid) in a given time, in the former 
inftance, is lefs than that which it phlogifticates, during 
an equal fpace of time, in the latter ; and the quantity of 
heat produced when a given portion of pure air is al¬ 
tered by the refpiration of an animal is nearly equal to 
that which is produced when the fame quantity of air is 
altered by the burning of wax or charcoal.” This ex- 
tenfive and important propofition is fupported by the 
refults of eighteen experiments. The firft confifts in 
immerfing a dog into a warm medium (water or air) for 
a due time, and then drawing a quantity of blood from 
an artery and a vein; when the venous blood was found 
nearly as florid a red as the arterial: but, when a dog had 
been confined for a time in a cold medium, the venous 
blood was of a much darker hue than in the former cafe. 
The fecond, third, fourth, fifth, fixth, and feventh, ex¬ 
periments, (how, that a fmall animal, fuch as a guinea- 
pig, requires more oxygen, and produces more carbonic 
acid, when placed in a cold than in a warm medium. 
The eighth fliows the quantity of heat produced by the 
combuftion of a given weight of wax, and the ninth the 
quantity of oxygen required, or which difappears in the 
procefs. The tenth (hows the quantity of heat produced 
by a guinea-pig in a given time, and the eleventh the 
quantity of oxygen fpent in the fame time. The twelfth 
ftiows the heat produced by the combuftion of a given 
weight of charcoal, and the thirteenth and fourteenth the 
quantity of oxygen which difappears, and is found in the 
carbonic acid. From this it appears, that the weight of 
the charcoal is to that of the oxygen as i to 2 nearly ; 
but more recent and accurate experiments of other 
authors find the ratio 1 to z '6 nearly. In the ^fifteenth 
experiment, wax was again burnt to determine whether 
aqueous vapour was produced in quantity, which ap¬ 
peared to be the cafe ; and the fixteenth and feventeenth 
go to prove, that in the combuftion of charcoal nothing 
but carbonic acid is produced. The eighteenth expe¬ 
riment feems to prove, that in the refpiration of a guinea- 
pig fome of the oxygen is fpent in producing aqueous 
vapour; but other experiments, fince made in more fa¬ 
vourable circumftances, do not warrant this conclufion. 
Colle&ing and comparing fome of thefe refults, it ap¬ 
peared that the heat communicated to 31 lb. 70Z. of water 
was as follow's : 
looouncemea-fby the combuftion of wax imparts 21 0 
fures of pure-jbythecombuftionofcharcoalimpartsi9 0 .3 
air altered . L by the refpiration of a guinea-pig i7°.3 
Upon a review of the preceding faffs and propolirions, 
it feems warrantable to conclude, ift, That the air infpired 
by animals contains more abfolute heat than the air ex¬ 
pired. 2dly, That the blood in the lungs receives the 
difference of heat, but without any material change of 
temperature, the capacity of the blood for heat being im¬ 
mediately enlarged juft fo much as to receive the ad¬ 
ditional heat without any change of temperature. 3dly, 
359 
That the blood, during the circulation through the body, 
gradually changes its capacity from a greater to a lei's, 
giving out heat to the body to fupply the wafte, till it 
again arrives at the lungs, where it parts with the char¬ 
coal, and receives more heat. 4thly, That the expen¬ 
diture of heat in an animal is proportional to the carbonic 
acid produced, or lofs of oxygen; and is accurately or 
nearly the fame as w'ould be given out by the combuftion 
of charcoal in a quantity fufficent to produce the fame 
acid. And 5thly, That the changes in the capacity and 
other qualities of the blood are greater, when an animal 
is placed in a cold than in a warm medium. Hence, if 
thefe conclufions be admitted, the fource of animal heat, 
and the mode of its diftribution to the various parts 
of the body, muff; be conlidered as fufllciently demon- 
ftrated. 
Although this explanation of the production of 
animal heat appears fimple and fatisfaflory, and has been 
held adequate to account for the phenomena, by fuch 
philofophers as Black, Crawford, Lavoifier, and La Place, 
the evidence on which it refts is not fo clear as to have 
commanded univerfal afienf, or entirely fet afide ob¬ 
jections. It has, indeed, been generally allowed, that 
refpiration and the changes it produces in the air and 
animal fluids, are eflential conditions of the evolution of 
the caloric in animals; but it has been thought, that 
there are other circumftances, hitherto perhaps not well 
underftood, which influence the phenomena. 
In external appearance the blood is the fame in all the 
vefiels of the fcetus ; is this any proof that its tempe¬ 
rature is owing to the converfion of oxygen gas into car¬ 
bonic acid ? 
The following inveftigations of Mr. Brodie are particu¬ 
larly calculated to increafe our doubts on the lubjeCf. 
Having pithed or decapitated rabbits, he kept up artificial 
refpiration, and thus maintained the circulation. The 
blood continued to be changed in the lungs from 
venous to arterial, and from arterial to venous, in 
the general circulation. The refpe&ive colours of 
the turn kinds of blood could not be diftinguiftied 
from thole which they exhibit in living and healthy 
animals. Yet the temperature of an animal thus 
heated, funk fafter than that of another Amply killed 
and left to itfelf. Thefe fafts, however, lead to no con¬ 
clusive theory on the fubjetl. Independent of the 
circumftance that the phenomena obferved by Brodie are 
not uniformly met with in all animals, no one can explain 
how the nervous power generates heat; whether it does 
it in the lungs or elfevvhere; whether it produces it im¬ 
mediately, or fecondarily by exciting chemical aiftions. 
Indeed, though the fubjeft has been in the faireft way of 
inveftigation of late, we do not fee that our cruel expe¬ 
riments have added one jot to the knowledge which w~as 
gained by the cautious obfervations of Hunter. To 
prove this, we quote a paflage from that author. “ This 
power (of generating heat) feems to be a property in an 
animal while alive. In the moft perfect animals it is to 
preferve a ftandard heat; and, as they are moft commonly 
in an atmofphere colder than themfelves, they have molt 
commonly occafion to exert it ; and it is therefore a 
power only of oppofition and refiftance; for it is not found 
to exert itfelf fpontaneoufly and unprovoked; but muft 
always be excited by the energy of fome external frigori- 
fic agent, or difeafe; yet it is natural to fuch animals that 
this power fhouid be called forth, as will be obferved by- 
and-by. It does not depend on the motion of the blood , 
asJ'ome have fuppofed, becaufe it likewife belongs to ani¬ 
mals who have no circulation; and the nofe of a dog, 
which is always nearly of the fame heat in all tempera¬ 
tures of the air, is well fupplied with blood; although 
we muft allow, where this power is greateft, the circula¬ 
tion is the quickeft. Neither can it be faid to depend 
upon the nervous fyftem ; for it is found in animals that 
have no brain or nerves. However, it muft be allowed, 
that all that clafs who pollefs this power in the highett 
5 degree, 
