460  Metrical  Weights  in  Prescriptions.  {Amuctur'I8P76arm' 
ably  that  no  reliable  rule  can  be  given  for  their  relative  proportion  of 
weight  to  volume.  But,  with  this  exception,  the  task  is  comparatively 
easy,  if  it  is  remembered  that  for  all  practical  purposes  one  gram  equals 
15  grains — that  two  tablespoonfuls,  /.  1  fluidounce,  of  water  weigh 
455*69  troy  grains,  or  (within  less  than  8  grains)  precisely  the  same  as 
30  grams  (463  grains).  15  grams  of  water  or  its  equivalent  in  bulk  of 
other  liquids  is,  therefore,  to  be  taken  as  equal  to  the  tablespoonful ; 
7*5  to  ^  grams  of  water  or  its  equivalent  to  the  dessertspoonful;  and 
3*7  to  4  grams  to  the  teaspoonful. 
Applying  these  values  to  the  heavier  and  lighter  liquids,  it  will  be 
seen  that — 
15  X  f  =  1 1 '25  grams  ether, 
15  X  f  =  1875  grams  glycerin, 
15  X  |  =  22-50  grams  chloroform, 
I5X|=I2      grams  spir.  aether,  comp.  (or  nitr.), 
l5  X  f  =  20      grams  syrup, 
are,  in  measure,  equal  to  about  half  a  fluidounce,  and  that  the  devia- 
tion from  this  measure  is  in  each  case  considerably  less  than  the  differ- 
ence in  the  amounts  obtained  by  scant  and  full  measurement  with  the 
same  tablespoon,  or  between  different  patterns  of  that  useful  domestic 
utensil. 
The  average  doses  of  these  liquids,  expressed  in  metrical  weights, 
are,  therefore — 
Ether  11.25  :  8  =  1*40  grams  or   \  teaspoonful. 
Spr.  seth.  comp  ,  and 
Spr.  asth.  nitr.,  12  :  4  =  3-00  "  or  1  " 
Chloroform,  22*50  :  8  =  2-80  "  or  \  " 
Glycerin,  1%'75  '■  4  =  47°     "     °r  1  " 
Syrups  (some),  20      -4  —  5  "     or  1  " 
For  the  conversion  of  grain  weights  of  solids  into  grams,  close 
approximations  to  the  correct  weight,  within  the  fraction  of  ^4,  are 
obtained  by  dividing  the  number  of  grains  by  15.  The  error  resulting 
from  this  rule  is  best  appreciated  by  comparing  the  results  obtained 
with  the  larger  weights,  thus  : 
Correct  Difference, 
weight.        Grams.  Grains, 
grs.  lx  give  -f-f  =   4-00        3-887        0*113  17436 
"    -4T85°-  =  32*00      31*100        0*90  13*89 
The  actual  difference  obtained  by  the  above  rule  amounts,  therefore, 
