248       Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
nents  contained  in  two  volumes  of  vapour);  specific  heat  of  non- 
volatile elements;  and  generalizations,  analogies,  and  conventions 
of  various  kinds,  occasionally  leading  to  higher  formulae  than 
those  deduced  from  the  convention  as  to  the  simplest  integral 
formula. 
When  fixed  in  these  ways,  several  remarkable  relations  be- 
tween the  formula?  of  bodies  and  their  physical  and  other  pro- 
perties are  found  to  exist.  Thus  a  number  of  crystalline  bodies 
of  analogous  and  similar  formulae  are  found  to  crystallize  in  forms 
of  the  same  geometrical  structure,  and  are  frequently  capable  of 
forming  different  portions  of  the  same  regular  geometric  crystal. 
Such  bodies  are  said  to  be  isomorphous ;  and  the  law  of  isomor- 
phism (Mitscherlich's  law)  may  be  simply  expressed  thus — 
similar  formula,  similar  crystal.  Thus  potassium  nitrate  and  cal- 
cium carbonate,  whose  formula?  are  analogous,  viz.  KNO3,  CaCO3, 
are  not  merely  isomorphous,  but  also  isodimorphous.  The  alums, 
the  magnesium-  and  copper- sulphate  families,  and  many  others 
might  also  be  cited. 
13.  The  following  connexion  is  found  to  exist  between  the 
specific  gravities  of  solid  and  liquid  substances  and  the  formula? 
attributed  to  them*  :  if  certain  numbers  be  assigned  to  each  ele- 
mentary symbol  in  a  formula,  these  numbers  sometimes  varying 
in  different  classes  of  compounds,  but  being  constant  for  all 
members  of  the  same  class,  the  sum  got  by  multiplying  each  of 
these  numbers  by  the  suffix  of  the  symbol  to  which  it  refers  and 
adding  together  the  products  is  always  approximately  equal  to 
the  quotient  obtained  by  dividing  by  the  specific  gravity  of  the 
body  in  question  (measured  under  certain  given  conditions,  as  at 
the  boiling-point  in  the  cases  of  liquids)  the  sum  obtained  by 
multiplying  the  combining  number  of  each  symbol  in  the  formula 
by  its  suffix  and  adding  together  the  products  thus  obtained. 
This  relation  is  more  simply  expressed  by  stating  that  the 
specific  volume  of  a  compound  can  be  calculated  when  its  formula 
is  known  together  with  certain  numbers  depending  only  on  the  nature 
of  the  component  elements,  and  in  some  instances  on  the  class  of 
compound  formed.  Thus  oxygen  has  the  number  7*8  attached 
to  it  in  compounds  analogous  to  water,  alcohol,  &c. ;  but  the 
number  12*2  in  such  bodies  as  aldehyde,  acetone,  &c.  Carbon 
has  similarly  the  number  11,  and  hydrogen  5*5.  Then  the 
specific  volume  of  water  at  its  boiling-point  is 
2  x  1 4- 16 
2  X  5*5  -f  7-8=18-8  ,which  approximately  =  , 
where  0*9579  is  the  specific  gravity  of  water  at  its  boiling-point. 
*  Kopp,  Ann.  der  Chem.  und  Pharm.  vol.  xcvi.  p.  153. 
