ON  TINCTURA  FERRI  CHLORIDI. 
19 
and  more  so  as  the  heat  is  raised,  and  the  watery  solution  does 
not  become  permanent  until  nearly  half  of  the  gas  has  escaped. 
Now  the  quantity  of  iron  dissolved  decreases  with  the  loss  of 
acid,  and  as  this  depends  on  the  solubility  of  the  iron  prepara- 
tion, some  requiring  a  much  more  prolonged  digestion  than 
others,  we  have  consequently  every  reason  to  fear,  that  this 
valuable  medicinal  agent,  will  be  found  to  vary  widely  in  its 
therapeutic  effects,  as  well  as  in  its  chemical  relations. 
In  the  formula  of  our  Pharmacopoeia,  there  is  no  provision 
made  to  compensate  for  this  variation  of  strength,  and  even  if 
half  the  acid  should  have  evaporated,  we  are  still  required  to 
add  three  pints  of  alcohol  to  the  remainder. 
I  have  already  shown  that  the  quantity  of  acid  in  the  for- 
mula, one  pint,  would  require  for  its  saturation  2446  grains  of 
hydrated  ox.  iron,  provided  there  was  no  loss  by  evaporation  and 
escape  of  gas.  On  this  latter  supposition,  the  whole  amount 
of  the  finished  tincture  would  measure  4  pints  or  64  fluid  ounces, 
and  this  would  give  38  grains*  in  each  fluid  ounce.  The  iron  is 
therefore  in  excess  434  grs.,  but  as  it  is  not  an  expensive  prepa- 
ration, it  might  be  safely  used  to  insure  against  accidental  im- 
purities, and  also  to  thoroughly  saturate  the  acid. 
A  much  more  important  and  practical  consideration  remains 
to  be  noticed,  namely,  how  to  provide  against  the  loss  of  acid, 
or  to  make  a  proper  compensation  for  it. 
The  remedy  suggested  will  fortunately  be  found  useful  in  a 
variety  of  cases,  several  of  which  have  occurred  in  my  own  expe- 
rience. It  is  briefly  this  :  After  digesting  the  subcarb.  of  iron  and 
acid  together,  until  the  acid  has  dissolved  all  it  is  capable  of,  allow 
it  to  settle,  and  pour  off  the  clear  solution,  or  filter  if  preferred  ; 
then  evaporate  the  solution,  with  a  gentle  heat  until  it  com- 
mences to  crystallize  upon  cooling ;  now  add  water  until  the 
specific  gravity  becomes  1-44,  when  cool.  If  now  found  to  be 
turbid  from  a  slight  deposit  of  sesquioxide  of  iron,  add  a  few 
drops  of  muriatic  acid,  and  warm  gently  till  clear  •  when  finished 
the  specific  gravity  should  be  1-44  at  60°  Fahr.  and  it  will  then 
*  According  to  Dr.  Squibb's  observation  there  is  a  contraction  in  volume 
when  the  alcohol  is  mixed  with  the  solution  of  iron,  of  about  2  per  cent., 
which  would  slightly  increase  the  calculated  strength  per  oz. 
