22      Action  of  Acids  upon  Zinc  Containing  Lead.  {^'^iaZ'Ssi:'''^' 
to  the  concentration  of  the  acid.  The  slight  variations  of  the  curves 
from  right  lines  are  due  to  the  differences  between  the  temperatures  of 
the  zinc  and  the  surrounding  liquid.  Measurements  of  the  electrical 
resistance  show  that  if  electrical  conductivity  exerts  any  influence 
during  the  period  of  induction,  it  is  without  any  sensible  effect  during 
the  second  part  of  the  reaction.  In  order  to  ascertain  the  iDfluence  of 
electrolysis  during  the  period  of  induction,  zinc  was  immersed  in  hy- 
drochloric acid  of  15  per  cent,  at  a  temperature  of  35°,  the  surface  of 
the  zinc  being  first  covered  with  a  thin  layer  of  gold,  platinum,  lead, 
or  copper.  If  the  period  of  induction  is  due  to  electrolysis,  the  rate  of 
solution  during  that  period  should  vary  with  the  electromotive  force 
of  the  couple  on  the  surface  of  the  zinc.  Possibly  also,  thermo-elec- 
tric currents  may  be  produced  by  the  difference  in  temperature  between 
the  zinc  and  the  surrounding  liquid,  or  by  differences  between  the 
temj)erature  of  different  parts  of  the  metal  itself.  Under  these  condi- 
tions, the  actual  initial  velocity  is  very  great,  but  gradually  diminishes 
until  it  reaches  a  minimum,  then  rises  and  attains  a  maximum,  and 
becomes  a  right  line,  indicating  that  the  rate  of  solution  is  propor- 
tional to  the  concentration  of  the  acid.  It  is  evident  from  this  result 
that  the  period  of  induction  is  the  time  during  which  the  acid  by  slow 
action  produces  at  the  surface  of  the  metal  an  infinite  number  of 
minute  galvanic  couples  by  exposing  the  particles  of  lead  which  are 
disseminated  throughout  the  zinc.  The  great  diminution  in  the  rate 
of  solution  is  probably  due  to  the  fact  that  the  liberated  hydrogen  re- 
moves mechanically  the  particles  of  the  foreign  metal  Avhich  had  been 
precipitated  on  the  surface  of  the  zinc.  The  actual  velocity  at  the 
commencement  of  the  reaction  increases  in  the  following  order : 
copper,  gold,  platinum,  lead  ;  whilst  the  theoretical  initial  velocity  in- 
creases in  the  order,  copper,  lead,  gold,  platinum.  Neither  of  these 
series  follows  the  order  of  the  electromotive  force  of  the  couples  pro- 
duced, and  hence  it  is  evident  that  although  electrolytic  action  plays 
an  important  part  in  the  solution  of  a  metal  in  an  acid,  it  is  by  no 
means  the  only  determining  cause. 
No  sensible  variations  in  the  rate  of  solution  were  observed  when 
considerable  quantities  of  sodium  or  potassium  nitrate  or  sulphates 
were  added  to  the  liquid,  and  it  would  therefore  seem  that  the  velocity 
of  the  reaction  is  not  greatly  affected  by  the  internal  friction  of  the 
solution. 
The  influence  of  temperature  is  shown  by  a  curve  the  ordinates  of 
