152 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



the blood which was taken in the warm bath could readily be distinguished from 

 that which had been taken from the same vein before immersion, by those who 

 were unacquainted with the motives or circumstances of the experiment. 



To discover whether a similar change would be produced in the colour of the 

 venous blood in hot air, a dog at 102° was placed in air at 134°. In 10 minutes 

 the temperature of the dog was 104% that of the air being 130°. In 15 mi- 

 nutes the dog was 106°, the air 130°. A small quantity of blood was then 

 taken from the jugular vein, the colour of which was sensibly altered, being 

 much lighter than in the natural state. The effect produced by external heat on 

 the colour of the venous blood, seems to confirm the following opinion, which 

 was first suggested by my worthy and ingenious friend Mr. Wilson, of Glasgow. 

 Admitting that the sensible heat of animals depends on the separation of abso- 

 lute heat from the blood by means of its union with the phlogistic principle in 

 the minute vessels, may there not be a certain temperature at which that fluid is 

 no longer capable of combining with phlogiston, and at which it must of course 

 cease to give off heat ? It was partly with a view to investigate the truth of this 

 opinion that Dr. C. was led to make the experiments recited above. 



I shall now endeavour, (says Dr. C.) from the preceding facts, to explain what ap- 

 pear to me to be the true causes of the cold produced by animals when placed in a 

 medium, the temperature of which is above the standard of their natural heat. 

 In a work which I some time since laid before the public, having attempted to 

 prove, that animal heat depends on the separation of elementary fire from the 

 air in the process of respiration, I observed, thaf when an animal is placed in a 

 warm medium, if the evaporation from the lungs be increased to a certain de- 

 gree, the whole of the heat separated from the air will be absorbed by the 

 aqueous vapour. From the experiments on venous and arterial blood, recited in 

 the 3d section of that work, it appears, that the capacity of the blood for con- 

 taining heat is so much augmented in the lungs, that, if its temperature were 

 not supported by the heat which is separated from the air, in the process of res- 

 piration, it would sink 30°. Hence, if the evaporation from the lungs be so 

 much increased as to carry off the whole of the heat that is detached from the 

 air the arterial blood when it returns by the pulmonary vein will have its sensible 

 heat greatly diminished, and will consequently absorb heat from the vessels 

 which are in contact with it, and from the parts adjacent. The heat which is 

 thus absorbed in the greater vessels will again be extricated in the capillaries, 

 where the blood receives a fresh addition of phlogiston. If, in these circum- 

 stances, the blood during each revolution were to be equally impregnated with 

 this latter principle, it is manifest, that the whole effect of the above process 

 would be to cool the system at the centre, and to heat it at the surface ; or to 

 convey the heat to that part of the body where it is capable of being instantly 



