244       Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
There  are  exceptions  to  this  rule,  viz. : — compounds  of  arsenic 
and  phosphorus, which  contain  only  multiples  of  \  volume  of  phos- 
phorus or  arsenic  vapour  in  2  volumes  of  compound  vapour,  and 
not  multiples  of  1  volume;  also  compounds  that  dissociate.  The 
former  compounds  are  excluded  from  consideration  on  the  as- 
sumption that  the  bodies  examined  in  the  vaporous  condition 
under  the  names  of  phosphorus  and  arsenic  are  not  the  true  ele- 
ments, but  only  allotropic  modifications  of  them,  bearing  to  them 
an  analogy  similar  to  that  of  ozone  to  oxygen,  a  view  corrobo- 
rated by  the  facts  known  with  regard  to  sulphur  (§  8). 
5.  The  smallest  number  of  grammes  of  an  element  contained  in 
two  volumes  of  the  homogeneous  vapour  of  any  of  its  compounds  is 
termed  the  Combining  Number  of  the  element.  Thus  the  small- 
est number  of  grammes  of  carbon  contained  in  the  homogeneous 
vapour  of  any  of  its  compounds  is  12 ;  similarly  of  oxygen,  16 ; 
of  hydrogen,  1 ;  and  so  on.  12,  16,  and  1  are  therefore  called 
the  combining  numbers  of  carbon,  oxygen,  and  hydrogen  re- 
spectively. 
6.  Many  compounds  contain  in  two  volumes  of  vapour  a 
larger  number  of  grammes  of  some  of  the  components  than  the 
combining  number;  the  number  of  grammes  thus  contained, 
however,  is  always  found  to  be  a  multiple  of  the  combining  number. 
Thus  compounds  of  hydrogen  contain  2,  3,  4,  ...  grammes  of 
hydrogen  in  two  volumes  of  vapour,  but  no  intermediate  frac- 
tional numbers  of  grammes ;  so  of  carbon,  24,  36,  48, . . . 
grammes,  but  no  intermediate  numbers;  so  of  oxygen,  16,  32, 
48,  64, . . .  grammes,  but  no  intermediate  numbers.  This  is  re- 
ferred to  under  the  name  of  the  "Law  of  multiple  proportions" 
7.  The  following  convention  is  employed  to  indicate  symboli- 
cally the  composition  of  compounds.  A  letter  from  the  name  of 
each  element  (or  more  letters  than  one)  is  taken  as  a  symbol  to 
represent  not  merely  that  element,  but  also  a  relative  weight  of  the 
element  in  the  proportion  of  its  combining  number ;  thus 
H  means  a  relative  weight  of  1  part  of  hydrogen ; 
C  „  „  12  parts  of  carbon ; 
0  „  „  16  parts  of  oxygen. 
Compounds  are  indicated  by  the  juxtaposition  of  the  symbols 
of  the  component  elements.  When  two  volumes  of  the  vapour 
of  the  compound  contain  a  multiple  of  the  combining  number 
of  grammes  of  any  element,  this  fact  is  expressed  by  applying  a 
suffix  to  the  symbol  of  the  element,  the  value  of  the  suffix  being 
the  numerical  value  of  the  multiple  :  in  accordance  with  the  law 
of  multiple  proportions,  this  value  is  always  an  integer,  a  mul- 
tiple of  unity. 
Thus  water  is  expressed  by  the  compound  symbol  IPO1, 
