^°^"jan.^'i888™"}    Actiou  of  Acids  upon  Zinc  Containing  Lead.  21 
In  all  caseSj  the  evolution  of  hydrogen  is  not  the  most  rapid  at  the 
commencement  of  the  reaction,  that  is,  when  the  acid  is  strongest,  but 
the  velocity  of  the  reaction  increases  until  it  reaches  a  maximum,  and 
then  decreases  in  such  a  way  that  the  rate  of  solution  is  proportional 
to  the  concentration  of  the  acid,  and  the  portion  of  the  curve  beyond 
the  maximum  is  a  right  line  which  cuts  the  axis  of  the  abscissae  at  a 
point  corresponding  witli  complete  solution.  The  first  part  of  the 
reaction,  during  which  the  rate  of  solution  increases  whilst  the  con- 
centration of  the  acid  diminishes,  is  abnormal,  and  represents  2i  period 
of  induction.  If  the  second  part  of  the  curve  is  prolonged  back- 
wards, it  cuts  the  axis  of  the  ordinates  at  a  point  which  gives  the 
initial  velocity  on  the  assumption  that  there  is  no  period  of  in- 
duction. 
The  theoretical  initial  velocity  as  thus  determined  increases  with  the 
volume  of  the  acid  used,  the  surface  of  zinc  exposed  remaining  con- 
stant. Experiments  show  that  the  temperature  of  the  zinc  rises  more 
rapidly  than  that  of  the  surrounding  liquid,  the  difference  being 
greater  the  greater  the  mass  of  the  liquid.  This  increase  in  the  tem- 
perature of  the  metal  of  course  increases  the  rate  of  solution.  If  the 
mass  of  the  zinc  varies  whilst  the  exposed  surface  remains  constant, 
the  smaller  the  mass  the  higher  the  rate  of  solution,  the  greatest 
velocity  being  observed  with  spheres.  This  result  is  due  to  the  fact 
that  the  temperature  rises  more  rapidly  in  the  case  of  the  smaller 
masses.  In  the  case  of  the  more  concentrated  acids,  spheres  of  zinc 
were  employed,  and  this  introduces  a  further  complication,  since  the 
area  of  the  surface  is  continually  diminishing  as  the  action  proceeds, 
and  the  ratio  of  the  surface  to  the  mass  is  likewise  changing.  The 
correction  required  is  given  by  the  expression 
Metallic  zinc  has  no  action  on  boiling  solutions  of  zinc  chloride. 
The  experiments  were  made  at  three  different  temperatures,  15°, 
35°,  and  55°,  with  hydrochloric  acid  of  5,  10  and  15  per  cent.,  the 
strengths  of  the  other  acids  being  adjusted  so  that  they  were  equiva- 
lent to  the  hydrochloric  acid  solutions. 
With  the  three  different  concentrations  of  hydrochloric  acid,  the 
ratio  of  the  rate  of  solution  to  the  concentration  of  the  acid  is  not  the 
same  in  all  three  series  when  the  whole  period  of  solution  is  taken  into 
account ;  but  if  the  period  of  induction  is  eliminated  in  the  manner 
already  indicated,  then  the  rate  of  solution  in  all  cases  is  proportional 
