490 Dr. crawford on the Power that 
It follows, that the quantity of heat given off by the blood 
in the capillaries will not be equal to that which it had ab- 
forbed in the greater veflels, or pofitive cold will be produced. 
If the blood, for example, in its paflage to the capillaries, ab- 
forb from the greater veflels a quantity of heat as 30°, and if 
in confequence of its receiving a lefs impregnation of phlo- 
gifton than formerly, it give off at the extreme veflels a quan- 
tity of heat only as 20°, it is manifeft, that upon the whole a 
degree of refrigeration will be produced as ro°, and this caufc 
of refrigeration will continue to a<ft while the venous blood is 
gradually afluming the hue of the arterial, till the difference 
between them is obliterated ; after which it will ceafe to ope- 
rate. Thus it appears, that when animals are placed in a 
warm medium, the fame procefs which formerly fupplied 
them with heat becomes for a time the inftrument of pro- 
ducing cold, and probably preferves them from fuch rapid alte- 
rations of temperature as might be fatal to life. 
Upon the whole, the increafed evaporation, the diminution 
of that power by which the blood in the natural ftate is im- 
pregnated with phlogifton, and the conftant reflux of the 
heated fluids towards the internal parts, feem to be the great 
caufes upon which the refrigeration depends. Having found 
that the attraction of the blood to phlogifton was diminifhed by 
heat, it appeared probable, on the other hand, that it would be 
increafed by cold. To determine this, a dog at ioo° was im- 
merfed in water nearly at 45 0 . In about a quarter of an hour 
a fmall quantity of blood was taken from the jugular vein, 
which was evidently much deeper in its colour than that which 
had been taken in the warm bath, and appeared to me, as well 
as to feveral other gentlemen, to be the darkeft venous blood 
we had ever feen. 
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