MORE  POISON  IN  SODA  WATER. 
307 
the  nitric  acid,  boiling  for  a  minute  or  so  after  each  addition, 
until  the  nitric  acid  ceases  to  produce  a  black  tint  in  the  liquor. 
The  complete  change  of  the  proto-sulphate  of  iron  to  a  per- 
sulphate, is,  towards  the  last,  accompanied  by  a  violent  boiling 
and  evolution  of  deutoxide  of  nitrogen ;  rendering  it  neces- 
sary to  use  a  vessel  for  the  operation  capable  of  holding 
double  the  quantity  put  into  it.  Boil  the  solution  of  per- 
sulphate of  iron  to  about  half  its  volume ;  then  add  the 
sulphate  of  ammonia,  and  set  it  aside  for  spontaneous  evapora- 
tion and  crystallization.  Wash  the  crystals  rapidly  but  tho- 
roughly in  very  cold  water,  then  press  them  wrapped  in  copious 
folds  of  bibulous  paper,  and  afterwards  dry  them  in  the  open  air. 
The  crystals  should  be  nearly  colorless,  but  with  a  slight  dark 
tint.  When  perfectly  formed,  they  are  octohedrons,  but  are 
are  usually  truncated.  Further  experience  may  possibly  dictate 
some  improvement  in  the  above  proportions,  but  they  are  suffi- 
ciently near  to  produce  good  results. 
Philadelphia,  6th  mo.  9th,  1856. 
ADDENDUM  TO  «  MORE  POISON  IN  '  SODA  WATER.'  " 
On  page  102  of  the  present  volume  of  this  Journal,  will  be 
found  a  brief  article  from  my  pen,  intended  to  show  the  presence 
of  lead  and  tin  in  "  soda  water ; "  but  especially  the  more 
poisonous  metal,  lead.  For  besides  the  chlorides  of  tin,  I  be- 
lieve our  toxicologists  do  not  acknowledge  any  poisonous  oxide 
or  salt  of  this  metal.  Notwithstanding  the  existence  of  tin  in 
the  carbonated  water  was  but  a  secondary  matter  of  concern,  yet 
the  presence  of  that  metal  was  inferred  under  circumstances  so 
adverse  to  the  deductions  from  the  common  routine  of  analysis, 
that  I  was  induced  to  say  in  the  article  alluded  to :  "  I  may  re- 
fer to  the  reactions  of  ferro-cyanide  of  potassium  and  tin  more 
particularly  at  another  time." 
The  subject,  however,  escaped  my  attention,  until  it  was  re- 
called by  a  letter  from  a  correspondent,  (though  personally  a 
stranger  to  me,)  of  Lowell,  Mass.  After  the  perusal  of  my 
article,  already  cited,  he  proceeded  to  analyse  some  of  the  car- 
bonated waters  of  the  shops  of  that  city.  He  found  the  speci- 
men examined  to  be  «  strongly  impregnated  with  copper  and 
lead]  "  but  no  indications  of  tin  presented  themselves.    To  ob- 
