of  Animals-  and  Vegetables.  3 1 
for  whenever  the  furrounding  cold  brings  them  to  that 
degree,  the  power  of  generating  heat  takes  place  till  life 
is  gone,  then  the  animal  freezes,  and  is  immediately 
capable  of  admitting  any  degree  of  cold. 
From  thefe  circumftances  of  thofe  imperfect  animals 
(upon  which  I made  my  experiments)  varying  their  heat 
fo  readily,  we  may  conclude,  that  heat  is  not  fo  very 
elfential  to  life  in  them  as  in  the  more  perfect ; although 
it  be  elfential  to  many  of  the  operations,  or  what  may 
be  called  the  fecondary  actions  of  life,  fuch  as  digefting 
food^,  and  the  propagation  of  their  fpecies,  which  re- 
quires the  greateft  power  an  animal  can  exert,  more  ef- 
pecially  the  laft ; and,  as  molt  of  the  more  perfect  of 
thefe  imperfedt  animals  are  commonly  employed  in  the 
firft,  we  may  fuppofe  their  heat  to  be  fuch  as  this  adtion 
of  life  requires,  although  in  them  it  be  never  eflentially 
neceflary  to  be  fo  high  as  to  produce  propagation  (c[. 
Therefore 
(h)  How  far  this  idea  holds  good  with  fifh  I am  not  certain. 
(c)  How  far  the  animal  heat  is  lowered  in  the  more  perfed  animals,  when 
thefe  fecondary  actions  are  not  neceflary.  as  in  the  bat,  hedge-hog,  bear,  &c. 
I have  not  been  able  to  determine,  not  having  opportunities  of  examining  thefe 
animals  in  their  involuntary  flate.  Dormice  are  in  a mixed  hate  between  the 
voluntary  and  involuntary,  and  we.  And  the  heat  diminished- when  the  actions 
are  not  vigorous;  and  from  a general  review  of  ? his  whole. fubjed  it  would 
appear,  that  a certain  degree  of  heat  in  the  animal  is  neceflary  for  digeftion, 
and  that  neceflary  heat  will  be  according  to  the  nature  of  the  animal.  A frog 
will  digeft  food  when  its  heat  is  at  6o°,  but  not  when  at  ,35°  or  40°;  and  it  is 
very 
