254      Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
number  of  grammes  of  chlorine  'which  the  combining  number 
of  grammes  of  the  radical  can  directly  or  indirectly  replace. 
Thus  0  in  PCI3  0  is  Bivalent,  its  equivalent  in  the  reaction 
PCP  +  H20=2HC1  +  PC130, 
being  8;  and  -—  =2. 
o 
CH  is  Trivalent  in  NCH  because  it  replaces  H3  in  NH3. 
Similarly  C  is  said  to  be  Quadrivalent  in  CH4,  this  compound 
being  viewed  as  formed  by  the  reaction 
4H2+C2  =  CH4  +  CH4, 
where  C  replaces  H4  in  4H2;  and  P  is  said  to  be  Quinquivalent, 
in  PCI5,  this  compound  being  producible  by  the  reaction 
5C12  +  P2=PC15+PC15. 
22.  The  same  symbol  or  aggregation  of  symbols  is  sometimes 
found  to  exhibit  different  valencies  in  different  compounds  :  in 
most  instances,  however,  the  alteration  in  valency  occurs  by  even 
differences ;  i.  e.  a  radical  is  always  either  an  Artiad  or  a  Perissad 
(Odliug). 
In  the  formulae  of  non -volatile  bodies,  apparent  exceptions  can 
always  be  made  to  conform  to  the  rule  by  employing  a  higher 
formula ;  in  the  case  of  the  few  volatile  bodies  which  apparently 
do  not  conform  to  this  rule,  dissociation  may  in  some  instances 
be  supposed  to  occur,  so  that  the  irregular  formula  is  perhaps 
incorrect. 
(\ 
OH 
OAg4 
SH2 
Sl'(C2H7 
IH 
ICP 
I02(OH)i 
Hg 
Hgl2 
Pt 
ptci2 
Ptci4 
PtK2  CI6 
CO 
CH4 
C2H2 
C2H4 
C2H6 
Fe2 
Fe2Cl4 
Fe2Cl6 
N(OAg) 
NO 
NH3 
NO2 
NH4  CI 
N(CB")*CP  I 
VC14 
VOC13 
WC11 
wc 
In  the  above  examples  it  is  noticeable  that  the  valency  of  the 
symbols  0,  S,  I,  Hg,  Pt,  C,  C2,  Pe2  always  alters  by  an  even 
number.  The  symbols  N,  V,  W  are  exceptions  to  this  rule; 
in  the  case  of  nitrogen,  however,  there  is  experimental  proof 
that  the  vapour  which  yields  the  formula  NO2  either  has 
dissociated,  or  has  undergone  some  change  analogous  to  disso- 
ciation, inasmuch  as  the  vapour  when  examined  under  different 
conditions  of  pressure  and  temperature  is  found  to  indicate  the 
formula  N204*.     It  might  be  supposed  that  nitric  oxide  has 
Playfair  and  Wanklyn,  Cbem.  Soc.  Journ.  [1]  vol.  xv.  p.  156. 
