Animals pojfefs of producing Cold. 49 . r 
From this experiment, compared with thofe which have 
been recited above, we may perceive the reafon why animals 
preferve an equal temperature, notwithstanding the great varia- 
tions in the heat of the atmofphere, arifing from the viciffi- 
tudes of the weather, and the difference of feafon and climate : 
for, as loon as by expofure to external cold, an unufual diffipa- 
tion of the vital heat is produced, the blood, in the courfe of’ 
the circulation, begins to be more deeply impregnated, with the 
phlogistic principle. It will therefore furnifh a more copious 
fupply of this principle to the air in the lungs, and will imbibe 
a greater quantity of fire in return. 
In fummer, on the contrary, the reverfe of this will take 
place, lefs phlogifton will be attracted in the minute veflels, 
and lefs fire will be abforbed from the air. 
And hence the power of generating heat is in all cafes pro- 
portioned to the demand. It is increafed by the winter colds, 
diminifhed by the fummer heats : it is totally fufpended or con- 
verted into a contrary power, according as the exigencies of the 
animal may require. 
From the changes which are produced in the colour of the 
venous blood by heat and cold, we may likewife perceive the 
reafon why the temperature of the body is frequently increafed 
by plunging fuddenly into cold water, and why the warm bath 
has fuch powerful effe&s in cooling the fyftem, and in removing 
a general or partial tendency to inflammation. 
