456  Contributions  from  the  School  of  Pharmacy,  etc,{^'^-^'^^'-J!^^''^' 
agent.    The  difficulty  in  obtaining  benzole  of  required  standard,  and 
the  trouble  of  rectifying  it,  as  well  as  the  superior  purity  of  the  gutta 
percha  precipitated  from  bisulphide  of  carbon,  are  the  considerations  in 
favor  of  the  latter. 
After  many  trials  with   varied  proportions   and    conditions,  the 
following  directions  for  preparation  of  the  solution  of  gutta  percha  from 
commercial  gutta  percha  are  presented  : 
(i.)  For  purification  of  the  gutta  percha:  In  a  strong  bottle  provided 
with  a  finely- ground  stopper,  and  containing  four  troyounces  bisulphide 
of  carbon,  place  120  grains  commercial  gutta  percha  (or  one  part  gutta 
percha  to  sixteen  of  the  solvent),  and  shake  frequently  until  dissolved. 
After  solution  has  taken  place,  add  a  small  quantity  of  animal  charcoal, 
shake  thoroughly  and  filter  under  a  bell  jar  placed  upon  a  ground  glass 
plate  (the  rim  of  the  jar  being  coated  with  tallow),  to  secure  an  air- 
tight vessel.  Into  a  wide  mouth  bottle,  provided  with  a  good  stopped" 
and  containing  four  volumes  of  alcohol  for  one  of  the  filtrate,  pour  the 
filtrate  a  little  at  a  time,  shaking  after  each  addition ;  then  shake 
thoroughly  until  the  precipitate  collects  into  one  mass  and  the  liquid  is 
quite  clear.  If  the  liquid  does  not  become  nearly  clear  after  shaking, 
add  more  alcohol  and  shake  again.  Pour  off  the  liquid  into  another 
bottle  ;  transfer  the  precipitate  to  a  pill-tile  and  press  it  with  a  spatula 
into  a  sheet  as  thin  as  possible,  and  leave  the  sheet  for  24  hours  or 
until  all  smell  of  bisulphide  of  carbon  and  alcohol  has  disappeared  and  a 
little  piece  from  the  thickest  part  of  the  sheet  dissolves  clear  in  chloro- 
form. (To  save  time,  the  precipitate,  instead  of  being  pressed  into  a 
sheet  may  be  stirred  in  an  evaporating  dish  on  the  water-bath  till  found 
pure  as  above.) 
(2.)  For  recovery  of  solvent  and  precipitant :  Place  the  liquid  from  the 
precipitate  in  a  retort  previously  set  over  the  water-bath  and  connected 
with  an  ice-cooled  receiver,  and  by  gentle  heat  distil  over  all  the 
bisulphide  of  carbon.  When  the  retort  is  cool,  remove  the  receiver 
and  add  to  the  bisulphide  of  carbon  a  large  quantity  of  water  and  shake 
gently;  pour  ofF  as  much  water  as  possible  and  then,  pouring  into  a 
burette,  draw  off  the  bisulphide  of  carbon,  from  the  remaining  water, 
throuo-h  a  filter  into  the  bottle  to  contain  it.  The  alcohol  in  the  retort 
should  be  filtered,  and  both  solvent  and  precipitant  are  ready  for  use 
again. 
(3.)  For  preparation  of  the  solution:  Take  of  the  fully  purified  gutta 
percha,  in  thin  slices,  forty-two  (42)  grains ;  chloroform  one  fluidounce  j 
