458 
Metrical  Weights  in  Prescriptions. 
Am.  Jour.  Pharm 
Oct.,  1876. 
It  may  be  remarked  in  this  place  tbat  the  "Pharmacopoeias"  of 
Continental  Europe  and  the  prescriptions  of  physicians  in  those  coun- 
tries express  all  quantities  by  weight  only,  whether  the  material  directed 
be  solid  or  liquid.  The  greater  exactness  of  gravimetric  over  volu- 
metric measurement  needs  scarcely  any  argument,  if  the  variation  in 
volume  under  the  influence  of  temperature  is  considered,  and  the  diffi- 
culty of  exact  measurement  in  glass  vessels  of  large  diameter  is  taken 
into  account.  Moreover,  weighing  is  more  convenient,  and  those  who 
have  accustomed  themselves  to  this  method  will  only  reluctantly  change 
it  for  measuring  again,  if  compelled  to  do  so. 
From  long  custom,  physicians  are  apt  to  over-estimate  the  difficulties 
of  writing  prescriptions  for  liquid  medicines  by  weights  instead  of 
measures.  Medicines  can  be  given  in  absolutely  definite  doses  only, 
if  divided  by  the  apothecary  ;  but  their  division  by  him  is  impracticable 
in  case  liquids  are  employed,  and  the  familiar  tea-,  dessert-  and  table- 
spoons are  then  resorted  to  for  dividing  the  medicines  at  the  bedside. 
How  widely  these  approximate  measures  differ  from  each  other,  and  to 
what  extent  the  difference  is  increased  by  the  manner  of  measuring 
with  them,  is  well  known.  The  apportioning  of  doses  of  liquid  med- 
icine would,  therefore,  offer  no  greater  difficulty  as  long  as  these  conve- 
nient and  handy,  but  variable,  measures  must  be  employed  in  the  sick- 
room. 
Of  the  officinal  liquid  preparations  which  are  prescribed  for  internal 
use,  the  alkaline  solutions  (of  soda,  potassa  and  ammonia),  the  diluted 
acids,  and  the  solutions  of  some  salts  (ammonium  acetate,  potassium 
citrate,  etc.),  do  not  differ  materially  in  bulk  from  an  equal  weight  of 
distilled  water,  this  difference  being  less,  particularly  for  the  quantities 
representing  their  medicinal  doses,  than  the  variations  of  the  popular 
approximate  measures  by  which  liquid  medicines  are  taken.  Tinctures 
and  fluid  extracts  vary  to  a  great  extent  in  density,  not  only  from 
water,  but  likewise  among  themselves.  The  apportioning  of  their 
doses  by  weight,  however,  would  be  an  easy  matter  if  the  resolution 
of  the  National  Convention  above  referred  to  had  been  carried  out  \ 
for  a  given  weight  of  the  preparation  would  then  represent  a  definite 
weight  of  the  drug,  and  the  proportion  of  the  weight  of  the  drug  to 
that  of  the  preparation  would,  for  tinctures,  most  likely  be  I  :  5  or 
1  :  10,  the  standard  generally  adopted  in  Europe.  Our  present  tinc- 
tures made  with  alcohol  and  diluted  alcohol,  in  the  proportion  of  two 
