Symbols  of  the  Elements. — In  expressing  the  composition  of  dif-
ferent chemical  substances  which  are  made  up  of  various  elements,
it  is  necessary,  as  a  matter  of  convenience  and  simplicity,  that  an  abbre-
viated form  of  chemical  language  be  employed.  This  has  been  provided
for  through  a  system  of  symbols,  in  which  the  initial  letter  of  the  title
of  the  element  is  generally  selected.*  In  some  cases  several  elements
possess  the  same  initial  letter;  it  is  then  the  custom  to  assign  the  single
letter  to  the  most  important,  abundant,  or  earliest-discovered  member
of  the  group,  and  to  the  others  a  second  letter.  Thus,  ten  names  of
elements  begin  with  C.  This  symbol  was  selected  for  Carbon,  as  the  most
important;  then  Ca  for  Calcium,  Cd  for  Cadmium,  Ce  for  Cerium,  CI  for
Chlorine,  Co  for  Cobalt,  and  so  on.  In  a  few  instances  the  symbols  are
selected  from  the  old  Latin  titles  of  the  elements.  Thus,  Fe  is  taken
from  Ferrum  (iron),  Pb  from  Plumbum  (lead),  K  from  Kalium  (potas-
sium), Au  from  Aurum  (gold),  and  so  on.
Atomic  Weight,  or  Mass. — The  above  symbols  possess  a  quanti-
y.  tative  signification.  Each  one  represents  one  atom  of  the  element  in
question,  this  being  the  smallest  quantity  of  an  element  which  is  present
in  the  molecule  of  its  compounds.  Thus,  Na  does  not  represent  any
indefinite  quantity  of  sodium,  nor  CI  any  amount  of  chlorine  These
symbols,  like  those  of  all  the  other  elements,  invariably  refer  to  definite
and  fixed  quantities,  as  in  the  above  instance  to  22.88  parts  of  sodium
and  35.18  parts  of  chlorine,  so  that  NaCl,  representing  a  molecule  of
sodium  chloride,  always  denotes  58.06  parts,  the  sum  of  one  atom  of
sodium  and  one  atom  of  chlorine.
