428 
ON  THE  DROP  AND  MINIM  MEASURE. 
I  have  lately  observed  the  tolerably  rapid  evaporation  of  the 
metal  with  steam,  produced  not  from  a  mass  of  water  covering 
the  quicksilver,  but  from  a  porous  and  highly  absorptive  clay 
which  was  dry  to  the  touch,  but  gave  off  some  eight  or  ten  per 
cent  of  water  when  heated  to  100°  C.    A  specimen  of  this  soil 
had  been  in  contact  with  mercury,  and  several  grams,  of  the 
metal  in  small  globules  had  become  mixed  up  with  the  mass.  It 
was  placed  in  the  common  copper  box  with  double  sides  which 
serves  as  a  steam-bath,  and  exposed  to  the  temperature  of  boil- 
ing water.    In  half  an  hour  I  was  surprised  to  see  that  the  little 
piece  of  glass  tube  through  which  the  heated  air  and  vapor  from 
the  inside  of  the  steam-bath  escapes  was  coated  with  a  bright 
specular  deposit  of  metallic  mercury.    This  was  brushed  off, 
and  the  tube  replaced,  when  again  it  became  in  a  short  time 
similarly  coated.    The  steam-bath  was  kept  in  constant  use  in 
drying  this  and  other  specimens  of  soil  for  twenty  or  thirty 
hours,  and  in  this  time  between  four  and  five  grains  of  mercury 
was  collected  from  the  glass  tube.    Doubtless  the  condensation 
of  the  mercury  vapor  on  the  latter  must  have  been  far  from  com- 
plete—a large  proportion  of  mercury  probably  escaped  into  the 
atmosphere.    At  any  rate  it  seems  clear  that  the  metal  can 
volatilize  in  very  considerable  amount  when  surrounded  by  vapor 
of  water  at  100°  C,  and  not  at  the  same  time  pressed  upon  or 
affected  by  the  cohesion  of  a  mass  of  liquid  water.    Hence  an 
obvious  necessity  for  thoroughly  effective   condensation  when 
mercury  is  t$  be  determined  in  its  compounds  by  ignition  with 
soda-lime. — Silliman's  Journal,  July,  1860. 
ON  THE  DROP  AND  MINIM  MEASURE. 
By  Mr.  Barnard  S.  Proctor. 
What  is  meant  by  "gtt."  ? 
This  is  a  question  which  the  dispenser  must  often  ask  himself, 
and  find  difficulty  in  deciding  upon  any  fixed  answer  which  will 
serve  all  occasions,  or  in  deciding  upon  any  rule  which  will 
guide  him  as  to  which  of  two  or  three  meanings  he  should  at- 
tach to  the  term  as  various  occasions  arise. 
It  has  been  more  than  once  pointed  out  that  a  drop  varies 
much  in  size  according  to  the  form  and  size  of  the  vessel  from 
