746  Chemical  Elements  of  Living  Matter,  j  A%J0°"r-J21Jarm- 
metabolism  of  the  cell  organism,  not  as  free  elements,  but  as  com- 
pounds. These  compounds  we  speak  properly  of  as  food,  of  which 
there  are  the  four  great  classes  of  proteins,  carbohydrates,  fats,  and 
salts.  The  first  three  classes  all  contain  hydrogen,  carbon,  oxygen, 
and  sometimes  nitrogen.  These  four  elements  are  of  paramount 
importance  in  all  life  processes,  and  the  multitude  of  compounds, 
which  they  are  able  to  form,  is  the  main  study  of  biochemistry.  To 
indicate  the  service  of  these  elements  in  the  living  organism  is  the 
aim  of  this  article,  although  present  knowledge  is  far  from  com- 
plete.  The  importance  of  the  elements  is  shown  in  the  three  groups : 
a.  The  true  life  elements  or  bioelements  which  are  invari- 
ably present  in  all  vegetable  or  animal  matter; 
b.  Elements  which  commonly  occur  in  cell  organism,  and 
which  seem  to  be  characteristic  of  certain  tissues ; 
c.  Elements  occurring  rarely  in  the  living  organisms,  which 
seem  to  be  more  or  less  adventitious. 
THE  TRUE  LIFE  ELEMENTS  OF  BIOELEMENTS. 
i.  Carbon,  well  known  as  the  structural  element  which  gives  to 
the  large  number  of  organic  compounds  their  complexity  of  form 
and  function,  is  the  corner-stone  of  organic  cell  metabolism.  Its 
valency  of  four  is  significant,  as  it  explains  the  fact  that  an  atom  of 
carbon  has  to  another  carbon  atom  a  greater  affinity  than  to  an 
atom  of  another  element.  In  other  words  the  attractive  power  of  a 
carbon  atom  is  greatest1  to  another  carbon  atom  for  its  electromo- 
tive force  is  nearly  zero,  it  can  act  therefore  sometimes  as  positive, 
sometimes  as  negative  atom  and  produces  the  many  chains  and  rings 
of  organic  compounds.  According  to  the  latest  theories  of  chem- 
istry, the  stable  compounds  all  contain  eight  or  a  multiple  of  eight 
valence  electrons,  these  alone  forming  a  stable  system.2  Carbon, 
having  four  valence  electrons,  combines  therefore  readily  with  other 
carbon  atoms  to  an  octet  or  stable  system  of  eight  electrons.3  The 
recognition  of  this  ability  of  the  carbon  atom  will  explain  its  pre- 
1  Geoffrey  Martin,  Ueber  das  Affinitaetsgesetz  im  Per.  Syst,  Inaug.  Diss., 
Kiel,  1906;  page  23. 
2  Lewis's  theory  of  cubical  atom,  Parson's  theory  of  magneton,  Langmuir's 
octet  theory  and  others. 
3  E.  g.f  sodium  has  1  free  valence  electron,  chlorine  has  7,  thus  NaCl  has 
together  8  valence  electrons.  Hydrogen  has'  1,  in  CH4  there  are  thus  8,  in 
C2H6  16  or  (2x8)  valence  electrons. 
