AV<^8£m'}  The  Metrical  System  in  Prescriptions.  53 
which  are  entirely  impracticable  for  the  dispensing  of  most  medicinal 
liquids  ;  and  that  the  probable  errors  of  reading  from  the  graduated 
measures,  as  usually  constructed  for  pharmacists,  are  much  larger  and 
increase  with  the  quantity,  must  be  evident  to  every  intelligent  observer. 
To  these  considerations  must  be  added  some  other  very  important 
ones,  namely,  the  great  volatility  of  some  liquids  at  ordinary  tempera- 
tures, and  the  tenacious  adhesion  of  others  to  the  graduated  vessels. 
The  dispensing  of  liquids  by  weight  offers,  for  all  the  reasons  advanced, 
by  far  greater  accuracv  than  could  be  attained  by  measures,  even  if 
they  were  constructed  upon  the  same  principle  as  burettes  and  pipettes  ; 
and  that  the  difficulties  of  dispensing  by  weight  are  not  greater  than  by 
the  use  of  measures  will  readily  be  acknowledged  by  those  who  have 
accustomed  themselves  to  the  former  practice,  and  this,  it  seems  to  me, 
will  become  the  duty  of  American  apothecaries  in  the  near  future. 
The  physician  now  in  practice  will  encounter  some  difficulties  in 
adapting  his  knowledge  and  experience  to  the  change  under  considera- 
tion, and  to  aid  him  in  this,  and  show  that  it  must  not  be  considered 
an  impossible  task,  even  with  our  present  "  Pharmacopoeia,"  was  one 
of  the  main  objects  of  my  previous  paper;  after  the  metric  system 
shall  have  been  fully  recognized  in  medicine  and  pharmacy,  the  stu- 
dents of  both  medicine  and  pharmacy  will  learn  the  proper  doses  of 
all  drugs  and  preparations  by  weight,  and  consequently  prescribe  and 
dispense  them  thus. 
A  common  oversight  by  many  physicians  may  here  be  incidentally 
alluded  to,  namely,  the  fact  that  solids  dissolved  in  liquids  occupy  a 
certain  space,  depending  in  part  upon  their  own  specific  gravity  and 
upon  the  nature  and  quantity  of  the  solvent.  On  the  other  hand,  it 
is  frequently  overlooked  that  on  mixing  certain  liquids  a  contraction 
takes  place,  as  in  the  case  of  alcohol  or  concentrated  acids  with  water. 
No  uniformly  applicable  rule  can  be  given  for  these  occurrences,  and 
in  most  cases  the  difference  in  the  expected  measure  falls  within  the 
variations  of  the  approximate  measures  to  which  the  patient  has 
recourse.  For  the  salts  of  the  alkalies,  alkaline  earths  and  even  the 
lighter  metals,  it  may  be  assumed  that  in  solution  they  occupy  the 
space  of  about  one-third  their  weight  of  water.  This  is  not  absolutely 
correct,  but  it  is  very  convenient  and  sufficiently  approximate  for  cal- 
culating the  dose.  Dr.  Blodgett  gives  a  formula  for  potass,  brom., 
grm.  12  *,  syr.  simpl.,  aq.  font.,  da  cc.  60,  which  it  seems  was  expected 
