512        Dr.  C.  R.  A.  Wright's  Reply  to  "An  Examination 
hydride,  the  two  resulting  monochlorides  are  (Schorlemmer) 
CH2C1,     CH2,     CH2,     CH2,     CH2,    CH8 
and 
CH8,     CHC1,     CH2,     CH2,     CH2,    CH3; 
i.  e.  one  represents  a  primary,  the  other  a  secondary  alcohol. 
What  are  the  differences  in  the  changes  of  motion  that  ensue  on 
the  replacement  of  a  hydrogen  atom  by  a  chlorine  atom,  according 
as  this  atom  is  one  of  the  terminal  or  of  the  penultimate  group  ? 
and  why  is  not  one  of  the  more  central  groups  affected  ?  So  far 
from  the  atomic  hypothesis  giving  an  explanation  of  chemical 
phenomena  relating  to  isomerism,  the  explanation  of  the  vast  ma- 
jority of  facts  by  this  means  has  never  yet  been  even  dreamed  of. 
It  might  be  fairly  urged  against  the  symbolic  system  that  it 
does  not  at  present  include  in  the  meanings  of  the  symbols  the 
amount  of  energy  converted  from  the  potential  to  the  actual 
state,  or  vice  versa,  in  any  given  reaction ;  but  this  is  a  point 
out  of  the  cognizance  of  my  paper,  which  only  treated  of  those 
things  that  are  represented  by  these  symbols. 
It  is  sometimes  stated  that  the  existence  of  atoms  (or  at  least 
of  molecules)  is  demonstrated  by  the  consideration  of  the  me- 
chanical properties  of  gases  and  other  physical  facts :  but  is 
there  not  a  petitio  principii  here  ?  Granting  the  existence  of  mo- 
lecules, certain  observed  properties  of  gases  may  be  explained, 
just  as  the  generalization  of  multiple  proportions  is  explained  by 
this  supposition ;  but  that  is  no  proof  of  the  existence  of  mole- 
cules. Granting  their  existence,  from  certain  physical  conside- 
rations their  size  may  be  approximately  calculated ;  but  that  is 
no  proof  of  their  existence,  any  more  than  the  circumstance  that 
certain  electric  phenomena  are  explainable  by,  and  certain  results 
deducible  from,  the  hypothesis  of  the  existence  of  an  imponde- 
rable fluid  is  a  proof  of  the  existence  of  such  a  fluid. 
When  it  is  urged  in  favour  of  the  retention  of  such  terms  as 
atom,  &c,  that  the  advances  in  chemistry  during  the  last  forty 
years  are  due  to  the  atomic  theory,  another  fallacy  is  introduced. 
It  does  not  follow  that  because  a  wire  is  useful  in  keeping 
a  young  sapling  erect,  therefore  the  wire  will  be  beneficial 
when  the  plant  is  strong  enough  to  dispense  with  its  aid ;  nay, 
the  wire  is  prejudicial,  as,  if  retained,  it  warps  the  tree  from  the 
perpendicular  and  causes  it  to  grow  distorted.  But  besides  this 
the  question  arises,  What  is  meant  by  the  term  atomic  theory  in 
this  case  ?  If  it  be  used  to  designate  what  has  been  styled  in 
my  paper  as  the  atomic  hypothesis,  the  truth  of  the  proposition 
may  be  doubted,  at  least  in  very  many  cases.  Thus  it  is  ques- 
tionable whether  the  chemists  who  employ  Kekule's  views  in 
their  researches  on  aromatic  compounds  use  the  symbolic  repre- 
