Atomic  Hypothesis  and  Dissected  (Structural)  For  mules.     257 
sensibly  to  detract  from  the  extreme  usefulness  of  dissected  for- 
mulae as  deduced  from  and  briefly  representing  a  large  number 
of  generalizations,  laws,  and  reactions,  as  if  memoria  technica" 
recalling  the  essential  chemical  differences  exhibited  between 
isomeric  substances  and  the  analogies  between  bodies  of  the  same 
classes,  and,  in  fine,  as  representing  in  extremely  small  compass, 
and  calling  to  mind,  all  the  salient  points  of  chemical  philosophy 
generally. 
26.  The  relations  of  the  atomic  hypothesis  to  the  facts  previ- 
ously mentioned  as  summed  up  in,  and  referred  to  by,  the  sym- 
bolic expressions  termed  dissected  formulae  may  be  briefly  stated 
thus  : — 
The  atomic  hypothesis  supposes  that  the  forces  known  to  us 
at  present  are  incapable  of  carrying  the  mechanical  or  chemical 
divisions  of  elementary  matter  beyond  a  certain  limit — that  in 
the  case  of  all  elementary  substances  small  portions,  indivisible 
by  these  forces,  exist,  these  portions  being  accordingly  termed 
atoms  :  by  the  union  together  of  these  primary  atoms  compound 
groups  of  atoms  termed  molecules  are  formed,  which  molecules, 
as  a  rule,  do  not  consist  of  any  large  number  of  component 
atoms.  The  relative  weights  of  the  ultimate  elementary  atoms 
are  denoted  by  the  combining  numbers  of  the  elements,  hence 
termed  atomic  weights ;  thus  the  atom  of  carbon  is  12  times  as 
heavy  as  that  of  hydrogen,  that  of  oxygen  16  times  as  heavy, 
and  so  on. 
Homogeneous  compounds  consist  of  a  large  number  of  pre- 
cisely similar  molecules,  and  hence  must  always  exhibit  the  same 
composition. 
The  assumption  known  as  Avogadro's  law  is  made,  that  equal 
bulks  of  all  homogeneous  gases  and  vapours  contain,  when  mea- 
sured under  the  same  circumstances  as  to  temperature  and  pres- 
sure, the  same  number  of  molecules.  From  this  it  follows  that 
the  formulae  of  bodies,  as  previously  defined,  express  the  relative 
numbers  of  the  different  kinds  of  atoms  present  in  the  mole- 
cules. It  does  not  necessarily  follow  that  the  values  of  the  suf- 
fixes applied  to  the  symbols  in  a  formula  express  the  absolute 
numbers  of  the  atoms  of  each  kind  present :  thus  a  molecule  of 
water  contains  twice  as  many  atoms  of  hydrogen  as  it  does  of 
of  oxygen  ;  but  whether  2  or  2/i  atoms  of  hydrogen  are  present, 
is  not  known.  The  assumption  is,  however,  made  that  the  suf- 
fixes express  the  absolute  numbers  of  atoms  of  each  kind  present. 
Hence  a  simple  explanation  is  given  of  the  observed  fact  termed 
the  Law  of  Multiple  Proportions  :  the  compounds  hydrochloric 
acid,  water,  ammonia,  and  marsh-gas  consist  of  molecules  con- 
taining respectively  1,  2,  3,  and  4  atoms  of  hydrogen  united  with 
1  atom  of  chlorine,  oxygen,  nitrogen,  and  carbon  respectively. 
Phil.  Mag.  S.  4.  Vol.  43.  No.  286.  April  1872.  S 
