4-S 8 Dr. crawford on the Power that 
jugular vein, the colour of which was fenfibly altered, being 
much lighter than in the natural ftate. 
The effect which is produced by external heat upon the co- 
lour of the venous blood, feems to confirm the following opinion,: 
which was firft fuggefted by my worthy and ingenious friend* 
Mr. Wilson, of Glafgpw. Admitting that the fenfible heat of 
animals depends upon the feparation of abfolute heat from the 
blood by means or its union with the phlogiftic principle in the 
minute vefiels, may there not be a certain temperature at which* 
that fluid is no longer capable of combining with phlogifton, 
and at which it muft of courfe ceafe to give off heat ? It was 
partly with a view to inveftigate the truth of this opinion that 
I was led to make the experiments recited above. 
I fhall now endeavour, from the preceding fadts, to explain: 
what appear to me to be the true caufes of the cold produced 
by animals when placed in a medium, the temperature of which 
is above the ftandard of their natural heat. 
In a work which Ifome time ago laid before the public, hav- 
ing attempted to prove, that animal heat depends upon the repa- 
ration of elementary fire from the air in the procefs of refpira- 
tion, I obferved, that when an animal is placed in a warm 
medium, if the evaporation from the lungs be increafed to a 
certain degree, the whole of the heat feparated from the air 
will be abforbed- by the aqueous vapour. 
From the experiments on venous and arterial blood, recited 
in the third fedtiori of that work, it appears, that the capacity' 
of the blood for containing heat is fo much augmented in the 
lungs, that, if its temperature were not fupported by the heat 
which is feparated from the air, in the procefs of refipiration, it 
would fink 30°. Hence, if the evaporation from the lungs be fa 
much increafed as to carry off the whole of the. heat ..that is 
detached 
