GLYCERINE  A  SUBSTITUTE  FOR  OILS,  ETC.,  IN  OINTMENTS.  255 
Take  Terbasic  Nitrate  of  Mercury.  3j. 
Plasma,  §j.  rti 
A  very  elegant  and  useful  preparation  with  Iodide  of  Potas- 
sium is  made  by  dissolving  the  iodide  in  the  glycerine,  mixing 
in  the  starch,  and  heating  to  240p  as  above — the  proportions 
being  Pharmacopoeial : — 
Take  Iodide  of  Pottassium,  3ij. 
Glycerine,  f.  gij. 
Starch,  140  grs. 
Mix  as  above. 
The  analogue  of  the  Unguentum  Iodinii  Compositum  I  for- 
bear to  introduce,  as,  however  valuable  the  iodide  of  starch 
may  be,  the  preparation  would  cease  to  be  an  application  of 
free  Iodine. 
But  though  not  quite  Pharmacopoeial,  I  venture  to  add  one 
other  formula,  which  several  medical  gentlemen  in  my  neighbor- 
hood are  now  employing  with  considerable  satisfaction  in  certain 
affections  of  the  skin  : — 
"  Plasma  Petrolei," 
Take  Petroleum,  3j. 
Glycerine,  f.gj. 
Starch,  70  grs. 
Rub  the  starch  with  the  petroleum  till  quite  smooth,  add  gradu- 
ally the  glycerine,  and  heat  to  240°  as  above. 
I  need  scarcely  add,  that  for  the  production  of  a  good  result, 
good  materials  must  be  employed.  Bottles  sent  out  from  whole- 
sale and  manufacturing  houses,  labelled  "  Pure  Glycerine," 
and  charged  accordingly,  do  not  always  contain  what  their  labels 
declare.  Indeed,  my  experience  during  the  last  few  months, 
leads  me  to  think  that  really  pure  glycerine  is  not  to  be  met 
with  in  commerce.  Some  samples  called  pure  are  rich  in  lead, 
others  contain  chlorine,  most  are  diluted  with  water,  and  the 
best  I  have  been  able  to  procure  is  acid.* 
I  have  found  it  necessary,  therefore,  to  purify  even  the  best 
samples  (Price's),  by  digesting  them  for  several  days  with  pow- 
dered chalk,  allowing  the  latter  to  subside,  and  decanting. 
*To  detect  this  acidity,  it  is  necessary  to  dilute  the  glycerine  with  an 
equal  bulk  of  water.  Its  action  on  litmus  paper  is  then  sufficiently  de- 
cisive. 
