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the perfecStion of the anim^il, the natural heat proper 
to each fpecies, and to each age. It may alfo perhaps 
depend, in fome degree, on other circumftances not hi- 
therto obferved: for from experiment ii. and iii. upon 
dormice, I found that in thefe animals, which are of a 
conftitution to retain nearly the fame heat in all tempe- 
ratures of the air, it required the greateft cold I could 
produce to overcome this power; while in experiment 
IV. and V. this power in the toad and fnail, whofe na- 
tural 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 i o° or 15°:: 
and the fnail being the moft imperfedt of the two, its^ 
powers of generating heat were by much the weakefl.. 
That the imperfe6t animals will allow of a confiderable; 
variation in their temperature of heat and cold,, is proved 
by the following experiments. The thermometer be- 
ing at 45°, having introduced the ball by the mouth into., 
the itomachof a frog, which had beenexpofed to the fame 
cold, it rofe to 49°. I then put the frog into an atmo- 
fphere made warm by heated water, and allowed it to^ 
ftay there twenty minutes ; when,, upon introducing the." 
thermometer into the ftomach, it raifed. the quicklilver^ 
to 64°. But to what degree the more imperfedt ani- 
mals are capable of being rendered hotter and colder,, 
at one time then another, I have not been able to deter- 
mine. The torpidity of thefe animals in our winter is> 
|«:obably owing to the great change wrought in their 
temperature: 
