394  ON  SOME  POINTS  OP  CHEMICAL  THEORY,  ETC. 
fein ;  although  the  substances  themselves  are  derived  from 
plants,  belonging  to  four  distinct  botanical  families.  Chocolate 
furnishes  another  drink,  employed  like  coffee  and  tea.  This 
substance,  it  is  true,  is  devoid  of  caffein  ;  but  it  contains  theo- 
bromin,  a  principle  closely  allied  to  caffein. 
A  few  remarks  will  now  be  made  on  the  beautiful  chemical 
relations  which  subsist  between  plants  and  animals.  The  vital 
processes  of  vegetation  are  uniformly  those  of  deoxidation,  and 
the  liberation  of  oxygen  ;  while  those  occurring  in  animals  are  the 
reverse,  or  oxidation.  The  vital  processes  in  plants  consist  in 
building  up  complex  molecules,  by  means,  for  the  most  part,  of 
deoxidation  ;  the  same  processes  in  animals  are  characterized  by 
the  reduction  of  complex  molecules  to  more  simple  ones,  effected 
by  oxidation.  If  plants  alone  existed,  the  oxygen  of  the  atmos- 
phere would  be  increased  and  its  carbonic  acid  diminished,  and 
plants  would  perish  for  want  of  carbon.  If  animals  alone  ex- 
isted, the  oxygen  of  the  atmosphere  would  be  diminished,  and  its 
carbonic  acid  increased,  until  at  last  it  would  become  unfit  to 
support  animal  life.  By  this  wonderful  compensation,  effected 
by  the  vital  action  of  plants  and  animals,  the  atmosphere  is 
kept  unchanged  in  composition  ;  and,  while  plants  give  out  oxy- 
gen, animals  consume  it,  converting  it  into  carbonic  acid,  which 
in  turn  furnishes  part  of  the  food  of  plants  ;  they  appropriating 
the  carbon,  and  giving  out  the  oxygen,  to  be  returned  once  more 
to  the  atmosphere. 
Organic  chemistry  is  an  extremely  important  study  for  the 
medical  man.  The  chemical  examination  of  plants  throws  much 
light  on  the  vegetable  materia  medica,  and  is  often  rewarded  by 
the  discovery  of  the  essential  principles  of  our  medicines,  on 
which  their  activity  depends.  The  analysis  of  animal  parts  and 
products,  both  healthy  and  diseased,  furnishes  information, 
highly  important  in  the  diagnosis  and  treatment  of  disease. 
What  would  have  been  the  present  state  of  pathology,  let  me 
ask,  had  the  physician  been  deprived  of  analyses  of  the  blood 
and  urine  ?  It  will  not  be  saying  too  much  to  assert,  in  view 
of  the  analytic  precision  attained  at  the  present  day,  that  the 
chemical  study  of  the  animal  secretions  will,  for  the  future,  form 
the  surest  means  of  advancing  practical  medicine. 
The  labors  of  physiological  chemists  have  pretty  well  settled 
