Atomic  Hypothesis  and  Dissected  [Structural)  Formulae.     253 
chlorine : — 
Potassium   .     .     .  =2943 
Zinc =1427 
Copper  .     .     .     .  =   859 
Mercury       ...  =822 
Accordingly  metallic  copper  precipitates  mercury  from  its  chlo- 
ride, forming  copper  chloride ;  zinc  decomposes  this,  precipita- 
ting the  copper  just  dissolved  and  forming  zinc  chloride,  which 
in  its  turn  is  capable  of  decomposition  by  potassium. 
In  cases  where  equivalent  quantities  of  different  substances 
produce  approximately  the  same  amounts  of  heat  by  union  with 
a  given  weight  of  some  other  body,  it  is  often  noticeable  that 
none  of  the  first  substances  will  decompose  a  compound  of  one  of 
them  and  the  other  body;  thus  35*5  grammes  of  chlorine 
by  combination  with  32  of  zinc  evolve  50296  calories 
„  „  28  „  iron     „       49651       „ 
or  approximately  the  same.  Accordingly  it  is  found  that  metallic 
zinc  has  no  action  on  ferrous  chloride,  and  that  metallic  iron  has 
no  action  on  zinc  chloride. 
Similarly  O4  in  uniting  with  F3  evolves  265776  calories 
while         Hs  „  O3       „      271Q48       „ 
Experiment  shows  that  by  passing  steam  over  red-hot  iron,  or 
hydrogen  over  red-hot  magnetic  iron  oxide,  the  reciprocal  reac- 
tions 
Fe3      +4H20  =  4H2    H-Fe304 
Fe304  +  4H2     =4H20  +  Fe3 
take  place  according  to  circumstances.  The  effect  which  the 
different  physical  properties  of  bodies  have  in  inducing  reactions 
is  well  exemplified  in  the  decomposition  of  potassium  sulphate 
by  barium  hydrate ;  the  production  of  the  insoluble  barium  sul- 
phate gives  rise  to  the  evolution  of  a  further  quantity  of  heat, 
i.  e.  the  latent  heat  of  barium  sulphate ;  and  hence  the  reaction 
ensues,  although  no  action  might  be  anticipated,  as  barium  hy- 
drate and  potassium  hydrate  evolve  equal  numbers  of  calories 
with  a  given  quantity  of  sulphuric  acid,  so  far  as  the  heat  of  the 
reaction  alone  is  concerned. 
21.  A  radical  being  the  name  given  to  an  assemblage  of  sym- 
bols (one  or  more)  that  occurs  in  the  formula  of  each  of  two 
substances  derivable  from  one  another  by  double  decomposition, 
the  term  Valency  is  applied  to  the  radical,  to  indicate  the  quotient 
obtained  by  dividing  the  combining  number  of  the  radical  by 
its  equivalent  in  the  particular  reaction  in  question,  i.  e.  to  in- 
dicate the  number  of  grammes  of  hydrogen,  or  «£-£-  of  the 
