246       Dr.  C.  E.  A.  Wright  on  the  Relations  between  the 
from  other  compounds  of  the  element  that  do  not  dissociate. 
Thus  the  vapour- density  of  calomel  (dissociated)  indicates  the 
formula  HgCl ;  but  the  generalization  of  even  variation  in  va- 
lency (§  22)  indicates  double  this,  or  Hg2  CI2 ;  and  the  vapour- 
densities  of  sulphuric  acid,  phosphorus  pentachloride,  and  am- 
monium chloride  would  represent  the  combining  numbers  of 
sulphur,  phosphorus,  nitrogen,  and  chlorine  as  respectively  16, 
15*5,  7,  and  17*75 — L  e.  the  halves  of  the  minimum  numbers 
occurring  in  undissociated  compounds. 
Compounds  that  dissociate,  or  are  believed  to  do  so,  must  there- 
fore be  excluded  in  the  determination  of  the  combining  number  of 
an  element. 
10.  In  the  case  of  the  elements  Mercury,  Sulphur,  Selenium, 
Tellurium,  Bromine,  Iodine,  Phosphorus,  Arsenic,  Antimony, 
Bismuth,  Tin,  Osmium,  Zinc,  and  probably  others,  the  combi- 
ning numbers  deduced  from  the  gasefiable  compounds  of  these 
elements  are  found  to  be  approximately  inversely  proportional  to 
the  specific  heats  of  these  elements  in  the  solid  state,  or  in  other 
words,  specific  heat  x  combining  number  =±  constant  =  6#6 
nearly. 
A  similar  law  is  found  to  hold  with  some  of  the  gaseous  ele- 
ments ;  only  in  this  case  the  value  of  the  constant  is  different,  as 
might  be  expected  from  the  different  physical  conditions ;  thus 
the  permanent  gases  oxygen,  hydrogen,  and  nitrogen  give  the 
constant  3*4,  and  the  condensable  gases  chlorine  and  bromine- 
vapour  the  constant  4*3. 
Applying  this  principle  to  those  elements  which  do  not  yield 
gasefiable  compounds,  it  is  found  that  when  each  element  is  de- 
noted by  a  symbolic  letter  (or  letters),  and  each  symbol  is  made 
to  represent  a  relative  weight  of  the  element  it  indicates  approxi- 
fi.fi 
mately  equal  to  -~-  (where  S  is  the  specific  heat  of  the  solid 
element),  the  quantitative  composition  of  all  compounds  of  these 
elements  may  be  represented  by  collocations  of  symbols  and  suf- 
fixes similar  in  character  to  the  formulas  deduced  for  the  gasefiable 
compounds  from  their  composition  and  vapour-density,  and  spe- 
cially characterized  by  the  fact  that  the  suffix-values  are  always 
integers  and  rarely  high  numbers. 
The  term  "  formula  "  is  therefore  extended  to  the  collocations 
of  symbols  and  suffixes  that  thus  express  the  quantitative  com- 
position of  non-volatile  compounds,  the  term  "  combining  num- 
ber "  being  similarly  extended  to  the  relative  numbers  approxi- 
mately equal  to  -5-  • 
In  many  instances  it  happens  that  a  relation  between  the 
values  of  the  suffixes  applied  to  a  given  symbol  in  several  differ- 
