24      Action  of  Acids  upon  Zinc  Containing  Lead.    /Am.  jour.  Pharm. 
Jan.,  18a>(. 
The  results  with  the  three  haloid  acids  are  quite  differeot  from  those 
obtained  in  the  case  of  the  action  of  the  same  acids  on  marble  [loc.  cit), 
in  which  case  the  rate  of  solution  is  the  same  for  all  three.  The 
velocity  of  the  reaction  with  zinc  has  no  simple  relation  to  the  elec- 
trical conductivity  of  the  acids,  their  heats  of  neutralization,  or  the 
solubilities  of  the  salts  produced. 
With  sulphuric  acid,  the  action  is  very  slow,  and  the  rate  of  solu- 
tion could  not  be  measured  with  an  acid  corresponding  with  10  per 
cent,  hydrochloric  acid.  The  period  of  induction  lasts  for  several 
hours,  and  hence  the  metal  was  always  previously  covered  with  a  film 
of  precipitated  lead.  At  36°,  the  velocity  is  only  one  twenty-seventh 
of  that  observed  in  the  case  of  hydrochloric  acid.  It  is  possible  that 
the  reaction  is  not  of  the  same  kind  as  with  the  haloid  acids.  It  may 
be  that  the  formation  of  zinc  sulphate  in  this  way  is  almost  entirely  a 
phenomenon  of  electrolysis,  and  that  the  chemical  attraction  of  the 
acid  for  the  metal  is  not  the  determining  cause  as  with  the  haloid 
acids.  In  the  latter  case,  there  is  simple  substitution  of  the  metal  for 
the  hydrogen  of  the  acid,  whilst  the  formation  of  zinc  sulphate  may 
be  the  result  of  a  series  of  reactions,  such  as  Zn  —  HgSO^  =  ZnO  ^ 
H2  -f  SO3 ;  SO3  +  H2O  -f  Aq  =  H2SO4  Aq  ;  ZnO  -f  H^SO,^ 
ZnSO,  -f  H2O. 
Pure  zinc,  rubbed  on  the  surface  with  metallic  lead,  does  not  dissolve 
in  acids  with  a  velocity  similar  to  that  of  zinc  alloyed  with  lead,  or 
zinc  covered  with  lead  by  precipitation.  The  black  residue  left  on 
solution  of  the  zinc  containing  lead  is  pure  lead.  The  difference  in 
the  electromotive  force  in  these  cases  may  be  due  to  the  state  of  divi- 
sion of  the  lead,  or  possibly  the  black  substance  is  an  allotropic  modi- 
fication of  the  lead.  If  pure  zinc  is  rubbed  with  this  lead-black  by 
means  of  a  spatula,  it  becomes  more  soluble  in  acids.  Mercury  amal- 
g:unates  and  dissolves  the  lead-black,  and  this  is  probably  the  reason 
why  amalgamated  zinc  is  not  soluble  in  acids,  d' Almeida's  view  that 
amalgamated  zinc  is  as  soluble  as  ordinary  zinc,  but  has  the  property 
of  condensing  on  its  surface  a  layer  of  hydrogen  which  protects  it 
from  the  acid,  is  not  supported  by  any  evidence. 
Note  hy  Abstractor. — L'Hote  has  recently  shown  that  perfectly  pure 
zinc  does  not  decompose  water,  and  is  not  soluble  in  acids.  According 
to  Osmond  and  Werth,  impure  zinc  when  dissolved  in  acids  leaves 
graphitoidal  residues  of  complicated  composition.  In  one  case  the 
composition  of  the  residue  agreed  with  the  formula  Pb2Zn. 
