262       Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
mula,  "  not  only  permit,  but  even  compel  us  to  regard  it  from  the 
atomic  point  of  view.  We  cannot  adopt  the  atomic  symbol  and 
at  the  same  time  declare  ourselves  free  from  the  atomic  doctrine." 
The  attempt  has  been  made  above  to  show  that  the  ordinary 
symbols  do  not  necessarily  involve  the  atomic  hypothesis  at  all 
— that,  by  suitably  choosing  definitions,  the  symbol  may  be  em- 
ployed and  yet  the  mind  of  the  chemist  be  free  from  the  atomic 
doctrine. 
Wurtz*  defines  an  atom  as  "la  plus  petite  quantite  d'un  element 
quipuisse  exister  dans  un  corps  compose. comme  masse  indivisible 
par  les  forces  chimiques  ;  "  a  molecule  "  est  un  groupe  d'atomes 
formant  la  plus  petite  quantite  d'un  corps  simple  ou  compose,  qui 
puisse  exister  a  Fetat  libre,  entrer  dans  une  reaction,  ou  en  sortir/" 
These  definitions,  which  are  now  all  but  universally  accepted, 
have  the  inconvenience  of  allowing  the  term  "  atom  "  to  be  em- 
ployed in  a  twofold  sense,  which  leads  to  some  misapprehension. 
In  one  sense  the  atom  is  a  finite  portion  of  matter  of  given 
weight,  and  hence  possessed  of  dimensions  in  space,  mass,  and 
time  denoted  by  the  formula  LMT-2;  this  corresponds  to  the 
Daltonian  sense.  In  another  sense,  however,  the  atom  represents 
not  an  absolute  weight,  a  given  portion  of  matter,  but  a  proportion 
or  ratio,  i.  e.  a  pure  number  possessed  of  no  dimensions  in  space, 
mass,  or  time  :  thus  water  is  said  to  be  composed  of  two  atoms 
of  hydrogen  and  one  atom  of  oxygen,  because  the  numerical 
ratio  of  the  weights  of  oxygen  and  hydrogen  in  a  given -quan- 
tity of  water  is  that  of  2  to  16 ;  while  in  certain  derivatives  from 
water  (e:  g.,  caustic  soda)  the  hydrogen  and  oxygen  are  found  to 
coexist  in  the  ratio  1  to  16,  which  is  expressed  by  saying  that  a 
molecule  of  this  substance  contains  but  one  hydrogen  atom  to 
one  oxygen  atom.  Using  the  term  in  this  latter  sense,  or  in 
one  akin  to  it,  Dr.  Williamson  f  states  that  the  "so-called 
law  of  multiple  proportions  has  no  existence  apart  from  the 
atomic  theory ;  those  who  adopt  it  seem  not  be  aware  that  they 
are  using  the  notion  of  atoms ;  or  else  they  are  shy  of  mentioning 
it  |"  and  again  that  "  when  one  of  those  who  profess  to  disbelieve 
in  the  atomic  theory  has  ascertained  by  analysis  the  percentage 
composition  of  a  compound,  and  wants  to  find  its  formula,  he 
divides  the  percentage  weight  of  each  element  by  its  atomic 
weight.  He  seeks  for  the  smallest  integral  numbers  which  re- 
present the  proportions  of  atoms,  and  he  attributes  to  impurity 
of  his  sample,  or  to  error  of  analysis,  any  deviation  from  the 
atomic  formula  thus  obtained.  He  looks  to  the  reactions  of  the 
body  for  aid  in  constructing  his  atomic  formula,  and  controls 
his  analyses  by  considerations  derived  from  well-established  re- 
*  Legons  de  Philosophie  Chimique  (Paris,  1864),  p.  39. 
t  C.  S.  J.  [2]  vol.  vii.  pp.  339,  340. 
