Am.  Jour.  Pharm. 
Oct.,  1891. 
}    Solubility  of  Chemically  Pure  Zinc. 
503 
Papaverine.    Bluish  color ;  the  liquid  becoming  bottle-green,  dirty  yellowish 
green,  violet  blue,  then  red.    A  small  bluish  deposit. 
Pilocarpine.  Nothing. 
Solanine.  Canary  yellow,  and  then  brownish.  After  half  an  hour,  a  rose 
colored  ring,  and  after  three  hours  the  liquid  becomes  violet  red. 
Saponin.    Yellowish,  becoming  slightly  reddish.    (Reaction  not  distinct.) 
Senegin.    L4ght,  dirty  yellow.    After  three  hours,  liquid  reddish. 
Veratrine.  Indistinct  yellowish  color,  sometimes  with  a  green  tone.  After 
half  an  hour  yellow.  After  three  hours,  deposit  red  and  liquid  yellowish. 
(Reaction  indistinct.) 
— A.  J.  Ferreira  de  Silva,  Comptes  Rendus,  112,  1266  ;  Jour.  Anal, 
and  Applied  Ckern.,  Aug.  i8gi,p.  4.78. 
THE  CAUSE  OF  THE  SLIGHT  SOLUBILITY  OF  CHEMI- 
The  slight  solubility  of  pure  zinc  in  dilute  acids  is  usually- 
explained  by  supposing  that  the  solution  of  impure  zinc  is  in  reality 
due  to  the  electric  currents  set  up  by  the  contact  of  the  zinc  with 
the  impurities,  and  that  as  in  pure  zinc  no  such  currents  can  occur, 
it  remains  undissolved.  This  theory  does  not,  however,  account  for 
the  solubility  of  the  pure  metal  in  nitric  acid,  or  even  in  dilute  sul- 
phuric or  hydrochloric  acid  at  their  boiling  points,  and  the  author 
has,  therefore,  again  investigated  the  subject.  As  the  result  of  his 
experiments  he  finds  that  the  insolubility  of  the  pure  zinc  is  due 
simply  to  the  formation  of  a  condensed  layer  of  hydrogen  on  the 
surface,  which  then  prevents  the  further  action  of  the  acid.  In  the 
case  of  nitric  acid,  this  layer  is  oxidized  by  the  acid  as  it  forms,  and 
cannot  protect  the  surface  of  the  metal,  whilst  in  the  case  of  impure 
zinc,  the  hydrogen  is  evolved  from  the  surface  of  the  more  electro- 
negative impurities  according  to  the  usual  law,  thus  leaving  the  sur- 
face of  the  zinc  exposed  to  the  action  of  the  acid. 
The  experiments  were  made  with  chemically  pure  zinc  and  sul- 
phuric acid,  the  latter  being  diluted  with  20  parts  of  pure  water.  It 
was  proved  that  even  such  simple  means  as  brushing  the  surface  of 
the  zinc  caused  a  considerable  increase  in  the  quantity  dissolved. 
This  became  much  more  striking  when  the  reaction  was  allowed  to 
take  place  in  a  vacuum,  the  relation  of  the  mean  quantity  dissolved 
CALLY  PURE  ZINC  IN  ACIDS.1 
By  J.  M.  Weeren. 
1  Berichte,  24,  1785-1798  ;  Jour.  Chem.  Soc,  Sept.,  p.  983. 
