252        Dr.  C.  R.  A.  Wright  on  the  Relations  betvieen  the 
'& 
combining  number  of  each  elementary  symbol  in  the  radical  by 
its  suffix  and  adding  together  the  products  :  thus  the  combining 
number  of  CH3  is  12  +  3x1=15;  of  PCI8,  31+3x355  = 
137-5;  ofCH,  12  +  1  =  13,  and  so  on.  When  it  is  said  that 
13  parts  of  the  radical  CH  replace  14  parts  of  nitrogen,  or  that 
16  of  oxygen  are  replaced  by  71  of  chlorine,  or  that  137'5  parts 
of  the  radical  PCI3  replace  2  of  hydrogen,  or  that  35*5  of  CI  re- 
place 1  of  hydrogen,  reference  is  made  to  the  existence  of  such 
chemical  reactions  as  are  represented  symbolically  by  the  equa- 
tions (1),  (2),  (3)  above. 
18.  From  the  comparison  of  such  reactions  as  those  repre- 
sented above,  it  is  found  that  35*5  parts  of  chlorine  always  are 
replaced  by,  or  are  substituted  for,  1  part  of  hydrogen ;  that  16 
of  oxygen  replace  71  of  chlorine  or  2  of  hydrogen,  and  so  on. 
This  is  expressed  by  saying  that  35*5  parts  of  chlorine  are  equi- 
valent to  1  of  hydrogen ;  that  16  parts  of  oxygen  are  equivalent 
to  1\  of  chlorine  or  to  2  of  hydrogen,  i.  e.  that  8  of  oxygen  are 
equivalent  to  355  of  chlorine  or  to  1  of  hydrogen,  and  so  on. 
1,  8,  and  35  5,  . . .  are  then  said  to  be  the  equivalents  of  hy- 
13 
drogen,  oxygen,  and  chlorine  respectively;  similarly  —  is  the 
o 
equivalent  or"  the  radical  CH  in  the  equation  (3) ;  — ^ —  of  the 
radical  PCI3  in  equation  (2). 
In  a  number  of  instances  the  following  physical  law  is  found 
to  hold  : — the  passage  of  a  given  quantity  of  electricity  through  an 
electrolyte  causes  the  evolution  of  equivalent  quantities  of  the  radi- 
cals into  which  it  decomposes  the  electrolyte,  no  manner  what  be 
the  nature  of  the  electrolyte.  Thus  if  a  given  quantity  of  elec- 
tricity evolve  1  part  of  hydrogen,  it  would  evolve  8  of  oxygen, 
35*5  of  chlorine,  and  so  on,  if  made  to  electrolyze  compounds  of 
those  elements  respectively. 
19.  In  some  instances  the  following  rule  holds, — that  the  heat 
evolved  by  the  action  of  a  given  weight  of  a  substance  on  equivalent 
quantities  of  analogous  bodies  is  approximately  constant-,  thus 
almost  exactly  the  same  amount  of  heat  is  produced  by  the  action 
of  a  given1  weight  of  sulphuric  acid  on  equivalent  quantities  of 
each  of  the  hydrates  of  potassium,  sodium,  lithium,  thallium, 
barium,  calcium,  strontium,  and  magnesium*. 
20.  In  other  instances  it  is  noticeable  that  the  greater  the 
amount  of  heat  given  out  in  any  number  of  a  series  of  analogous 
reactions  involving  equivalent  quantities  of  the  substances  used, 
the  more  stable  is  the  resulting  product ;  thus  equivalent  quan- 
tities of  potassium,  zinc,  copper,  and  mercury  evolve  the  fol- 
lowing numbers    of  calories  when  they  unite  with  1   part  of 
*  Thomsen,  Deut.  Chem.  Ges.  Ber.  vol.  iv.  p.  3(R 
