7I4  Dissolving  Zinc  Chloride.  {^0™?'^™' 
So  much  for  our  study  of  zinc  chloride  under  the  conditions  of 
ordinary  manipulation  by  the  pharmacist ;  and  like  every  definite 
chemical  substance  it  always  behaves  the  same  under  identical  con- 
ditions no  matter  how  often  they  recur. 
Still  the  problem  how  to  get  zinc  chloride  into  solutions  of  such 
strength  as  ordinarily  called  for  in  prescriptions  without  loss,  re- 
mained  unsolved.  Left  as  we  were  to  make  the  best  of  what  cannot 
be  changed,  one  might  decide,  and  with  sanction  of  authority,  as 
previously  pointed  out,  to  prepare  a  solution  of  zinc  chloride  and 
ignoring  the  slight  precipitate,  ascribe  to  it  an  approximately  definite 
strength  and  of  this  take  sufficient  quantity  as  needed.  Such  course 
is  perhaps  the  next  best  plan  to  getting  the  zinc  salt  entirely  dis- 
solved, but  the  accomplishment  of  the  latter  was  what  had  prompted 
the  work ;  and  yet  meditation  seemed  to  be  but  closing  the  door  to 
possibility. 
While  reviewing  numerous  experiences  with  zinc  chloride  in  pre- 
scription work,  it  wras  thought  that  there  was  a  possibility  of  boric 
acid  being  the  agent  sought.  It  had  been  used  a  great  many  times 
in  eye-washes  with  zinc  chloride  but  had  never  been  depended  upon 
for  solvent  effect.  Its  suitability  to  eye  washes  and  its  mild  acidity, 
however,  strongly  urged  a  trial,  and  the  first  experiment  showed  that 
it  has  the  very  property  desired,  while  subsequent  trials  established 
the  fact  that  boric  acid  will  give  clear  solutions  with  zinc  chloride, 
grain  for  grain,  in  any  dilution,  hot  or  cold,  up  to  about  twTenty 
grains  per  ounce.  Boric  acid  applied  to  the  zinc  chloride  will  pre- 
vent the  precipitation  of  zinc  oxychloride ;  it  will  also  redissolve  a 
precipitate  of  oxychloride,  although  proportionately  more  is  then 
required  than  is  needed  to  prevent  the  precipitate.  A  saturated  so- 
lution of  boric  acid  is  a  convenient  form  in  which  to  employ  the 
agent,  adding  it  directly  to  the  zinc  chloride,  or  the  reverse.  Sat- 
urated solutions  of  boric  acid  at  ordinary  indoor  temperatures  con- 
tain about  twenty  grains  of  boric  acid  to  the  fluid  ounce ;  these 
solutions  will  dissolve  an  equal  amount  of  zinc  chloride.  If  such 
concentrated  solutions  of  zinc  chloride  are  exposed  to  a  lower  tem- 
perature, they  become  cloudy,  peculiarly  so,  and  though  cleared  by 
warmth  they  do  not  retain  their  transparency. 
Solution  by  means  of  boric  acid  may  also  be  accomplished  by 
mixing  the  boric  acid  intimately  with  the  zinc  chloride  before  the 
aqueous  solvent  is  applied.  This  plan  involves  some  allowance  for 
the  slowT  solubility  "of  the  boric  acid  itself.    Solutions  of  zinc  chloride 
