Animals pojfefs of producing Cold. 489 
detached from the air, the arterial blood when it returns by the 
pulmonary vein will have its fenfible heat greatly diminifhed, 
and will confequently abforbheat from the veffels which are in 
contafl with it, and from the parts adjacent. The heat which 
is thus abforbed in the greater veffels will again be extricated 
in the capillaries, where the blood receives a frefh addition of 
phlogifton. If, in thefe circumftances, the blood during each 
revolution were to be equally impregnated with this latter prin- 
ciple, it is manifeft, that the whole effedt of the above procefs 
would be to cool the lyftem at the center, and to heat it at the 
furface ; or to convey the heat to that part of the body where 
it is capable of being inftantly carried off by evaporation. But 
it appears, from the experiments which have been -laft recited, 
that, when an animal is placed in a heated medium, the fan- 
guineous mafs, during each revolution, is lefs impregnated with 
phlogifton ; for we have feen, that the venous blood, in thefe 
circumflances, becomes gradually paler and paler in its colour 
till at length it acquires very nearly the appearance of the ar- 
terial : and it is rendered highly probable by the experiments of 
Dr. Priestley, that the dark and livid colour of the blood in 
the veins depends upon its combination with phlogifton in the 
minute veffels. Since, therefore, in a heated medium, this fluid 
does not affume the fame livid hue, we may conclude, that it 
does not attraft an equal quantity of the phlogiftic principle 
* It is of no conference in the above argument, whether we fuppofe, with 
Dr. priestley, that the alteration of colour in the blood depends upon its com- 
bination with phlogifton in the capillary arteries, or maintain with fome other 
philofophers that this alteration arifes from a change produced in the blood itfelf by 
the action of the veffels ; it is fufficient for our purpofe to affume it as a facft, 
which, I think, has been proved by direct experiment, that, in the natural ftate 
of the animal, the blood undergoes . a change in the capillaries, by which its capa- 
city for containing heat diminifhed ; and that in a heated medium it does not 
undergo a fimjlar change. 
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