Amoc°tu,r'x8P7h6arm'}    Metrical  Weights  in  Prescriptions.  459 
troyounces  to  the  pint,  are  very  nearly  of  the  strength  1  :  6|  and  1  :  7 
respectively  ;  and  those  containing  three  troyounces  to  the  pint,  1  ;  \\ 
and  1  :  5.  Tinctura  opii  is  in  the  proportion  of  1  :  and  tinctura 
opii  camphorata,  1  :  250  (for  opium).  It  will  be  observed  that  the 
changes  necessary  in  the  officinal  formulas  to  bring  them  in  harmony 
with  the  metrical  system  are  by  no  means  as  great  and  revolutionary  as 
is  sometimes  supposed,  particularly  if  the  necessary,  or  at  least  desir- 
able, change  in  the  alcoholic  strength  of  the  menstruum  is  taken  into 
consideration. 
A  uniform  standard  of  strength  for  fluid  extracts  would  most  likely 
be  the  proportion  of  1:2;  or,  to  give  it  in  the  metrical  system,  5:10; 
for  the  proportion  1  :  1  is  unattainable  in  all  cases  where  sugar  or  much 
glycerin  is  requisite  for  preservation  ;  and  it  appears  to  be  desirable  to 
adhere  to  an  adopted  standard,  also,  for  those  in  the  preparation  of 
which  alcohol  or  stronger  alcohol  is  exclusively  employed. 
Of  the  remaining  liquid  medicines,  the  doses  by  weight  can  be  as 
easily  acquired  as  by  minims  or  fluidrachms  ;  but  for  those  who  have 
already  become  conversant  with  the  measures  now  in  use  here,  the 
following  observations  will  offer  all  requisite  facilities  for  converting 
them  into  weights.  Ether,  having  the  specific  gravity  '750,  occupies 
precisely  the  same  volume  as  \\  time  its  weight  of  water,  and  the  dif- 
ference in  the  volume  of  stronger  ether  (specific  gravity  '720)  is  con- 
siderably within  the  limits  of  variation  of  the  approximate  measures  5. 
or,  in  other  words,  three  parts  by  weight  of  ether  occupy  the  same 
space  as  four  parts  by  weight  of  water.  The  relation  of  the  weight 
of  spiritus  aetheris  compositus  (specific  gravity  '815)  and  spiritus  aethe- 
ris  nitrosi  (specific  gravity  '837)  to  volume  is  very  nearly  as  4  :  5,  that 
is  to  say,  four  parts  by  weight  of  the  preparations  named  occupy  (a 
little  less  than)  the  same  space  occupied  by  five  parts  of  water. 
Glycerin  (specific  gravity  1*25)  has  a  proportion  of  weight  to  vol- 
ume as  5  :  4  ;  syrups  (specific  gravity  1*317)  nearly  as  4:3;  chloro- 
form (specific  gravity  1*48)  nearly  as  3:2,  z.  \\  part  by  weight  of 
glycerin,  ij  part  of  syrup,  and  \\  part  of  chloroform,  occupy,  approx- 
imately, the  same  volume  as  1  part  of  water. 
If  the  relative  density  of  the  officinal  liquids  is  not  lost  sight  of,  it 
will  be  seen  that  there  is  no  great  obstacle  in  the  way  of  prescribing 
by  metrical  weights  even  the  preparations  of  our  present  "  Pharmaco- 
poeia," except  in  the  cases  of  fluid  extracts,  which  vary  so  consider- 
