30 
LIQUOR  FERRI  TERSULPHATIS. 
of  an  olive  green  color  in  the  very  dilute  solution,  add  a  little 
more  nitric  acid  and  repeat  the  boiling.  Finally  dilute  the 
solution  to  two  hundred  parts  and  filter. 
The  sulphuric  acid  should  be  added  to  the  water  before  heat- 
ing in  order  to  avoid  explosions  from  the  rapid  combination, 
and  when  the  nitric  acid  is  added,  if  the  basin  be  placed  over  a 
lamp  or  in  a  water  bath  the  heat  will  be  sufficient  to  start  the 
reaction,  and  the  sulphate  may  be  added  rapidly.  The  process 
should  be  conducted  under  a  hood  or  in  the  open  air,  and  if  the 
heating  be  brisk  enough  the  violent  reaction  is  finished  in  a  very 
few  minutes.  If  the  nitric  acid  be  of  the  officinal  strength,  there 
will  not  be  a  trace  of  proto  salt  discoverable  by  the  test  a 
moment  after  the  last  crystals  are  dissolved,  and  the  additions 
of  the  salt  in  the  large  crystals  in  which  it  usually  occurs  is  an 
advantage  to  the  process,  because  the  reaction  is  then  more 
moderate,  and  less  of  the  nitric  acid  is  driven  off  undecomposed  in 
the  vapors.  The  boiling  is  directed  for  the  purpose  of  driving  off 
oxides  of  nitrogen,  and  to  diminish  any  slight  excess  of  nitric 
acid.  It  need  not  be  continued  beyond  a  few  minutes,  and  the 
whole  process,  except  the  filtration,  even  when  ounces  are  taken 
for  the  parts  in  the  formula,  may  be  completed  within  an  hour. 
The  filtration  is  very  tedious,  and  not  always  necessary.  It 
need  never  be  applied  to  any  but  the  last  portions  of  a  bottle  of 
the  solution,  provided  time  be  allowed  for  the  insoluble  matter 
to  settle.  The  testing  should  be  performed  upon  a  single  drop 
of  the  solution  dropped  into  a  test  tube  half  filled  with  water, 
and  a  single  drop  of  solution  of  ferridcyanide  is  all  that  it  is 
proper  to  add.  Great  mistakes  are  often  made  in  the  applica- 
tion of  delicate  tests  by  using  too  large  quantities. 
The  process  is  simple  and  easy  and  needs  no  great  degree  of 
skill  for  its  performance,  the  principal  point  being  so  to  keep 
up  the  heat  that  the  effervescence  progresses  with  the  additions 
of  the  salt,  and  does  not  accumulate  till  near  the  end,  and  then 
froth  over  the  basin.  Such  an  accident  of  course  vitiates  the 
result. 
The  writer  has  used  this  formula  upon  the  various  scales  of  a 
quarter  of  a  pound,  an  ounce  and  a  drachm  to  each  part  of  the 
formula  and  with  the  same  convenience  in  all. 
The  resulting  solution  is  a  syrupy  liquid  of  a  dark,  dull  brown 
