HYDRARGYRUM  CUM  CRETA,  ETC. 
surfaces  of  the  globules  become  covered  and  hidden  by  the  fine 
powder  of  chalk,  so  that  though  large,  they  are  not  easily  seen. 
Such  specimens,  unless  artificially  colored,  are  of  a  very  light 
color,  and  upon  admixture  with  water  afford  in  a  moment  a 
nearly  white  mixture,  from  the  rapid  subsidence  of  the  mercury. 
On  the  other  hand,  when  the  trituration  is  too  long  continued, 
the  mercury  becomes  too  finely  divided,  and  in  this  condition 
oxidizes  too  rapidly  and  too  readily,  as  well  in  the  primae  viae 
when  administered,  as  during  the  process  of  preparation  and  in 
the  keeping.  In  the  trituration  of  the  materials  in  the  ordinary 
way  no  appreciable  oxidation  occurs  until  the  globules  become 
too  small  to  be  seen  by  the  naked  eye.  But  after  this  point  has 
been  passed,  the  rate  of  oxidation  increases  rapidly  as  the 
mercury  becomes  more  and  more  divided,  until  finally  it  may  be 
in  large  proportion  converted  into  sub-oxide  by  oxidation  from 
the  air.  The  art  of  making  a  good  preparation,  therefore,  con- 
sists in  excluding  the  air  wholly  or  partially  during  the  process, 
and  in  arresting  the  trituration  at  the  proper  time.  Very  finely 
divided  mercury  oxidizes  and  dissolves  very  easily,  becoming 
first  a  sub-oxide,  whilst  this  sub- oxide  in  its  turn*  is  still  more 
easily  converted  into  per-oxide  and  metallic  mercury.  Thus  the 
sub-oxide,  which,  under  ordinary  circumstances  is  among  the 
mildest  of  mercurial  compounds,  from  the  instability  of  its 
chemical  character  is  very  likely  to  react  with  the  vitiated 
secretions  of  cholera  infantum,  &c,  just  as  Mialhe  has  shown 
that  calomel  sometimes  does,  and  thus  be  easily  converted  into 
per-oxide  and  per-chloride.  That  this  is  the  proper  explanation 
of  the  undue  activity  of  some  samples  of  this  preparation,  is 
rendered  almost  certain  by  the  writers'  observations  of  the 
similar  changes  that  spontaneously  occur  when  such  specimens 
are  long  kept  with  free  access  of  light  and  air.  Thus,  a  speci- 
men prepared  almost  without  contact  of  air,  was  of  a  very  dark 
color,  and  nearly  free  from  oxidation.  At  the  end  of  a  year's 
exposure  to  light  in  a  loosely  stopped  bottle,  it  was  found  to 
contain  both  oxides  in  very  notable  proportion.  Another  portion 
prepared  by  trituration  with  steam  power  for  two  days  and  a 
half,  by  a  good  mechanical  arrangement,  but  without  any  attempt 
at  excluding  the  air,  was  found  at  the  end  of  the  trituration  to 
contain  a  notable  proportion  of  sub-oxide,  but  no  per-oxide. 
