64  PHARMACEUTICAL  APPLICATIONS  OF  GLYCERINE. 
compounds  differing  from  each  other  in  some  of  their  physical 
properties.     The  arrow-root  plasma  is  beautifully  transparent, 
but  has  an  objectionable  tenacity  if  more  than  sixteen  grains  to 
the  ounce  be  used.    On  the  whole,  I  prefer  the  tous-les-mois 
preparation  to  any  other.    The  best  mode  of  preparing  this  is, 
to  rub  together  in  a  mortar  fifty  grains  of  tous-les-mois  with  one 
ounce  of  glycerine  ;  transfer  this  to  a  porcelain  evaporating- 
dish,  and  heat  over  a  gas  flame  to  a  temperature  of  240°,  con- 
stantly stirring  with  an  ivory  or  wooden  spatula.    (A  prescrip- 
tion recently  came  under  my  notice,  in  which  an  eminent  surgeon 
had  ordered  starch  and  glycerine  to  be  heated  to  this  tempera- 
ture over  a  water-bath !  !)    Some  pharmaceutists  recommend 
that  the  plasma  should  be  kept  at  240°  for  twenty  minutes,  but 
I  see  no  advantages  likely  to  accrue  from  this,  and  unless  great 
care  be  taken  to  regulate  the  temperature  the  compound  will 
become  colored,  and  will  always  be  found  to  have  diminished 
considerably  in  weight,  a  result  not  at  all  satisfactory  to  the 
operator.    If  the  application  of  heat  be  continued  only  long 
enough  to  burst  the  starch  granules,  or  till  the  mixture  becomes 
transparent,  the  loss  will  be  about  twenty  grains  to  the  ounce. 
The  presence  of  a  little  water  is  not  detrimental ;  indeed,  I  be- 
lieve it  improves  the  condition  of  the  product,  as  it  will  be  found 
more  plastic  and  better  suited  for  rubbing  over  the  surface  of 
the  skin,  even  after  it  has  been  exposed  to  the  air  a  few  weeks, 
and  thus  absorbed  more  moisture.    M.  Surum,  a  French  phar- 
maceutist, who  has  paid  much  attention  to  the  subject,  advises 
ten  per  cent,  of  water  to  be  added  to  the  starch  previous  to  mix- 
ing it  with  the  glycerine.    I  do  not  think  plasma  would  be  ad- 
vantageously substituted  for  fats  in  all  ointments,  but  in  those 
cases  where  there  is  a  great  tendency  to  rancidity,  as  in  the 
Cer.  Calam.,  Cer.  Plumbi  Acet.,  Ung.  Zinci,  etc.,  of  the  old 
Pharmacopoeia,  and  where  the  active  ingredient  of  the  ointment 
is  soluble  in  glycerine,  as  in  the  Ung.  Potass.  Iod.,  Ung.  Aconi- 
tise,  Ung.  Atropise,  Ung.  Belladonna,  Ung.  Creosoti,  and  Ung. 
Yeratrise  of  the  British  Pharmacopoeia,  the  plasmas  appear  pre- 
ferable to  the  analogous  ointments  ;  it  also  has  the  advantage  of 
being  easily  removed  from  the  skin  without  the  aid  of  soap  or 
friction. 
Those  preparations  in  which  glycerine  alone  is  the  basis  have 
