Ana.  Jour.  Phartu.l 
March,  1903.  / 
Life-History  of  a  Doctrine. 
129 
But  when  two  univalent  atoms  unite  with  one  bivalent  atom  we  can 
hardly  avoid  thinking  of  two  points  of  contact  on  the  bivalent  atom 
and  of  two  directions  in  which  it  exerts  its  powers  of  combination. 
This  conception  of  direction  is  further  forced  upon  us  by  a  study  of 
the  phenomena  of  stereochemistry,  especially  in  the  field  of  the 
chemistry  of  the  compounds  of  carbon.  But,  if  the  carbon  atom 
exerts  its  powers  of  combination  in  definite  directions  that  can  be 
determined  by  observation,  it  is,  to  say  the  least,  highly  probable 
that  all  other  elements  act  in  the  same  general  way,  and  indeed 
many  facts  have  been  discovered  within  the  last  few  years  that  have 
given  a  clue  to  the  stereochemistry  of  nitrogen,  of  sulphur,  of  sili- 
con and  other  elements.  Indeed,  in  the  studies  of  Werner,  already 
referred  to,  stereochemical  phenomena  are  illustrated  in  many  ways 
by  compounds  of  platinum,  palladium  and  other  metals  that  enter 
into  the  complex  inorganic  bases. 
Our  imaginary  atom  then  has  mass.  It  has  the  power  to  com- 
bine with  other  atoms  under  the  proper  conditions.  This  power  is 
either  a  unit,  as  in  the  univalent  elements,  or  it  is  divisible  by  2,  3, 
4»  5»  6,  7  or  8  in  the  case  of  other  elements.  Further,  one  and  the 
same  element  may  exhibit  different  powers  under  different  condi- 
tions, but  the  laws  governing  these  variations  are  not  known. 
Finally,  the  powers  of  combination  of  a  polyvalent  atom  are  exerted 
in  definite  directions  that  can  to  some  extent  be  determined.  These 
directions  are  evidently  subject  to  variation,  and  some  effect  upon  a 
compound  caused  by  displacement  has  apparently  been  shown  in 
the  case  of  some  carbon  compounds ;  at  least  Von  Baeyer's  strain 
theory  is  based  upon  this  assumption. 
The  latest  turn  that  has  been  given  to  the  conception  of  the  atom 
brings  in  again  the  electric  charge.  It  appears  that  the  cotempora- 
ries  of  Berzelius  were  too  easily  frightened,  and  Berzelius  was  nearer 
right  than  they  supposed.  Every  book  on  the  history  of  chemistry 
has  an  obituary  on  the  electrochemical  theory  of  Berzelius.  But 
now  it  appears  that  the  electrical  charges  assumed  by  him  must  be 
assumed  by  us.  These  have  come  more  and  more  to  the  front  of 
late,  and  chemical  union  is  being  regarded  more  and  more  as  due  to 
the  interaction  of  these  charges.  According  to  the  modern  concep- 
tion, an  atom  may  or  may  not  be  carrying  a  charge  of  electricity. 
When  carrying  its  charge  it  is  called  an  ion,  and  it  is  then  ready  for 
action.    When  the  elementary  ion  gives  up  its  charge,  either  by 
