NOTES  ON  PRESCRIBING. 
343 
experiments  have  an  important  practical  bearing  on  the  art  of 
prescribing,  showing  that  medicines,  harmless  when  administered 
separately,  may  become  highly  deleterious  when  given  in  combi- 
nation. 
The  following  case  of  unexpected  change  in  the  composition  of 
a  medicine  was  of  actual  occurrence.  A  prescription  was  writ- 
ten for  a  mixture  of  which  the  more  essential  ingredients  were 
Rochelle  Salt  and  Calcined  Magnesia,  the  one  dissolved,  the 
other  diffused  in  peppermint  water.  The  mixture  was  prescribed 
and  taken  without  particular  remark,  until,  upon  one  occasion, 
recourse  was  had  to  a  bottle  which  had  been  prepared  some 
weeks  before.  The  dose  was  found  extremely  different  from  any 
that  had  been  taken  previously  :  in  fact  it  had  so  caustic  a  taste 
as  to  excite  the  alarm  of  the  patient,  who  suspected  a  serious 
error  on  the  part  of  the  druggist.  The  physician  was  consulted, 
and  finally  an  analytical  chemist  was  requested  to  examine  and 
report  on  the  medicine.  This  resulted  in  an  explanation  : — the 
Calcined  Magnesia,  by  prolonged  contact  with  the  alkaline  tar- 
trates, had  gradually  abstracted  their  tartaric  acid,  leaving  their 
alkalies  in  a  free  and  caustic  state. 
The  dispenser  of  prescriptions  is  sometimes  puzzled  to  know 
what  color  to  make  a  medicine,  the  color  being  dependent  on  the 
order  in  which  the  ingredients  are  mixed.  For  instance,  a  lotion 
was  prescribed  composed  of  calomel,  lime  water,  and  chloride  of 
zinc.  If  the  calomel  was  decomposed  first,  the  lotion  was 
black :  if  the  chloride  of  zinc  first,  it  was  white. 
Lotions  in  which  both  chloride  and  bichloride  of  mercury  are 
ordered  with  lime  water,  are  easily  made  to  vary  from  yellow  to 
brown  or  black,  according  to  the  order  in  which  the  two  mercu- 
rials are  decomposed.  A  lotion  made  according  to  the  follow- 
ing formula  is  either  transparent  and  colorless,  or  opaque  and  of 
a  brick  red,  according  to  the  order  in  which  the  ingredients  are 
mixed : 
R  Potassse  chloratis, 
Boracis  aa  3ss. 
Hydrargyri  bichloridi  gr.  iv. 
Glycerinae  Jss. 
Aquae  ad  Iviij. 
Misce. 
