VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 153 



carried off by evaporation. But it appears, from the experiments which have 

 been last recited, that when an animal is placed in a heated medium, the san- 

 guineous mass, during each revolution, is less impregnated with phlogiston ; for 

 we have seen, that the venous blood, in these circumstances, becomes gradually 

 paler and paler in its colour, till at length it acquires very nearly the appearance 

 of the arterial : 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 on 

 its combination with phlogiston in the minute vessels. Since therefore, in a 

 heated medium, this fluid does not assume the same livid hue, we may conclude, 

 that it does not attract an equal quantity of the phlogistic principle.* 



It follows, that the quantity of heat given off by the blood in the capillaries 

 will not be equal to that which it had absorbed in the greater vessels, or positive 

 cold will be produced. If the blood, for example, in its passage to the capil- 

 laries, absorb from the greater vessels, a quantity of heat as 30°, and if, incon- 

 sequence of its receiving a less impregnation of phlogiston than formerly, it give 

 off at the extreme vessels a quantity of heat only as 20°, it is manifest, that on 

 the whole a degree of refrigeration will be produced as 10°, and this cause of re- 

 frigeration will continue to act while the venous blood is gradually assuming the 

 hue of the arterial, till the difference between them is obliterated ; after which it 

 will cease to operate. Thus it appears, that when animals are placed in a warm 

 medium, the same process which formerly supplied them with heat becomes for 

 a time the instrument of producing cold, and probably preserves them from 

 such rapid alterations of temperature as might be fatal to life. 



On the whole, the increased evaporation, the diminution of that power by 

 which the blood in the natural state is impregnated with phlogiston, and the 

 constant reflux of the heated fluids towards the internal parts, seem to be the 

 great causes on which the refrigeration depends. Having found that the attrac- 

 tion of the blood to phlogiston was diminished by heat, it appeared probable, on 

 the other hand, that it would be increased by cold. To determine this, a dog at 

 100° was immersed in water nearly at 45°. In about a quarter of an hour a 

 small 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 



* It is of no consequence in the above argument, whether we suppose, with Dr. Priestley, that 

 the alteration of colour in the blood depends on its combination witli phlogiston in the capillary arte- 

 ries, or maintain with some other philosophers that this alteration arises from a change produced in 

 the blood itself by the action of the vessels ; it is sufficient for our purpose to assume it as a fact, 

 which, I think, has been proved by direct experiment, that, in the natural state of the animal, the 

 blood undergoes a change in the capillaries, by which its capacity for containing heat is diminished ; 

 and that in a heated medium it does not undergo a similar change. — Orig. 



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