Animals pojjefs of producing Cold . 4^1 
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 vicifii- 
tudes of the weather, and the difference of feafon. and climate : 
for, as foon 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 
phlogiftic 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 tak^ 
place,, lefs phlogrfton will be attracted in the minute vefifels, 
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, 
diminifibed 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 finch powerful effects in cooling the fyidem, and in removing 
a- general or partial. tendency to inflammation. 
