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ON  THE  PREPARATION  OF  CHLORIDE    OF  ZINC. 
I  then  add  the  precipitated  chalk,  as  being  purer  and  therefore 
preferable  to  ordinary  prepared  chalk,  and  proceed  to  evaporate 
to  dryness,  during  which  any  iron  that  may  be  present  separates 
as  oxyde.  The  impure  chloride  is  then  dissolved  in  distilled  or 
pure  filtered  water,  and  again  filtered,  and  again  evaporated. 
This  last  operation  requires  close  and  careful  watching.  The 
evaporation  is  continued  longer  than  is  generally  thought  neces- 
sary. With  a  very  gentle  heat,  the  directions  given  by  the 
Pharmacopoeia  to  evaporate  to  dryness,  may  be  complied  with, 
but  owing  to  the  exceedingly  deliquescent  nature  of  this  salt,  I 
think  this  an  ineligible  form ;  for  if  the  bottle  containing  the 
chloride  in  a  pulverulent  form  be  left  open  even  for  a  short 
time,  it  speedily  forms  a  coherent  mass,  and  becomes  inconve- 
nient for  use.  I  therefore  continue  the  evaporation  over  a  brisk 
fire  until  what  may  be  called  the  first  stage  of  evaporation  is 
completed,  which  may  be  known  by  dipping  into  the  melted  mass 
a  glass  rod.  If  complete,  a  white  opaque  pellicle  immediately  forms 
upon  the  rod,  as  soon  as  withdrawn.  This  may  with  propriety 
be  called  the  aqueous  fusion,  and  the  salt  may  be  poured  on  a 
marble  slab.  It  chills  very  quickly,  and  becomes  solid;  but  as 
this  did  not  possess  the  requisite  hardness  and  fineness,  the  heat 
was  continued  upon  another  portion,  until  it  fairly  passed  into 
the  igneous  fusion,  the  frothing  ceased,  and  the  melted  liquid 
became  beautifully  clear  and  of  a  high  sp.  gr.,  as  a  bit  of  broken 
glass  readily  floated  on  the  surface.  It  may  now  be  poured  on 
a  marble  slab  as  before,  and,  when  cool,  broken  to  pieces  and 
kept  in  well  stopped  vials  secured  by  wax.  As  thus  prepared, 
it  is  beautifully  white  and  hard,  though  equally  as  deliquescent 
as  when  prepared  in  any  other  way,  yet  presents  less  surface  to 
the  action  of  the  moisture  of  the  air. 
Before  closing  this  paper,  I  wish  to  make  a  few  remarks  upon 
the  crystalline  nature  of  this  salt,  as  I  do  not  remember  having 
met  with  any  previous  notice  of  it.  At  or  about  the  time  it  begins 
to  pass  into  the  aqueous  fusion,  if  a  strip  of  glass  be  dipped 
into  the  solution  and  carefully  watched  as  it  begins  slowly  to 
solidify,  the  crystallization  of  this  chloride  may  be  readily  seen. 
At  first  only  a  point  or  nucleus ;  it  then  steadily  increases  in 
size,  when  the  real  form  of  the  crystal  may  be  discerned, — 
acicular  and  radiant  from  one  centre.    This  is  conducted  with 
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