176  APPARATUS  FOR  TAKING  SPECIFIC  GRAVITY. 
30  dollars,  or  about  two  per  cent,  upon  the  value ;  an  amount  of 
error  to  which  such  specimens  are  liable,  with  any  apparatus. 
But  it  was  obvious  that  the  method  of  displacement  required 
a  series  of  vessels  specially  adapted  to  the  operation,  to  complete 
with  weighing  in  closeness  of  results.  After  a  good  deal  of  re- 
flection and  experimenting,  in  which  many  modifications  of  shape 
and  arrangement  were  tried  and  discarded,  and  which  it  would 
be  cumbersome  to  notice  in  this  place,  the  apparatus  now  shown 
was  found  to  answer  best.  In  practice  it  is  really  a  pleasant  and 
satisfactory  substitute  for  the  tedious  and  irksome  method 
usually  resorted  to.  Some  few  precautions  must,  of  course,  be 
attended  to.  The  vessel  must  stand  firmly.  If  at  first  the 
water  will  not  flow,  or  flows  fitfully,  the  obstruction  will  be  re- 
moved by  blowing  a  little  in  the  spout. 
An  investigation  of  some  interest,  growing  out  of  this  matter, 
may  properly  be  noticed.  Where  we  are  operating  on  substances 
of  low  specific  gravity,  say  wood  or  stone,  a  drop  or  two  of  water, 
or  the  size  of  the  drop,  in  tapering  off  the  divisor,  is  of  no  conse- 
quence. But  it  is  otherwise  in  the  case  of  a  gold  coin,  for  ex- 
ample : — in  a  double  eagle,  the  difference  of  one  drop  of  water 
(ordinary  about  a  half-grain)  in  the  devisor,  would  affect  the  re- 
sult to  the  extent  of  0-3,  which  carried  into  the  fineness,  would 
make  a  difference  of  15  or  20  thousandths  ;  and  in  the  case  of  a 
half-eagle,  the  uncertainty  of  result  would  be  proportionally  in- 
creased. The  question  then  arose,  what  fluid,  or  what  modifica- 
tion of  water,  will  afford  us  a  smaller  drop  ?  for,  as  was  just 
observed,  a  half-grain  is,  on  the  average,  the  smallest  of  clean 
water  that  will  detach  itself  by  its  own  weight.  Very  much 
depends,  of  course,  upon  the  size  of  the  aperture,  in  the  measure 
of  drops  of  fluid  ;  one  drop  of  water  from  a  large  beak  weighed 
1 J  grains.  In  the  Dispensatory  of  Drs.  Wood  and  Bache,  there 
is  a  table  of  the  experimental  results  of  Mr.  Durand,  showing 
the  number  of  drops  of  different  liquids  equivalent  to  afluidrachm 
(page  1405.)  The  differences  are  very  remarkable ;  distilled 
water,  for  instance,  being  set  down  at  45  drops,  and  pure  alcohol 
at  138  drops.  And  in  our  own  experiments,  the  drop  of  alcohol 
was  about  one-third  the  weight  of  the  drop  of  water,  from  the 
same  pipette.    This  seemed  to  point  to  alcohol  as  a  substitute ; 
