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powers, in doing this, in proportion to 
the perfection of the animal, the natural 
heat proper to each fpecies, and to each 
age.—It may alfo perhaps depend, in fome 
degree, on other circumftances not hitherto 
obferved; for, from experiments upon dor¬ 
mice, I found, that in thefe animals, 
which are of a conftitution to retain nearly 
the fame heat in all temperatures of the 
air, it required the greateft cold I could 
produce to overcome this power, while in 
other experiments, this power in the toad 
and fnail, whofe natural heat is not always 
the fame, but is altered very materially 
according to the external heat or cold, was 
exhaufted in a degree of cold not exceeding 
io° or 15 0 , and the fnail being the molt 
imperfeCt of the two, its power of gene¬ 
rating heat was by much the weakeft.— 
The imperfeCt animals will allow of a 
confiderable variation in their temperature 
of heat and cold.-—Plants, when in a ftate 
of 
