460 
FACTS  RELATING  TO  MAGNESIUM. 
by  characteristics  known  to  toxicologists,  and  which,  therefore, 
•it  is  unnecessary  to  mention  here. 
If  the  suspected  liquids  contain  no  trace  of  arsenic  or 
antimony,  they  may  contain  other  poisonous  metals,  such  as 
copper,  lead,  mercury,  zinc,  &c.  In  this  case  the  metals  are 
found  as  flakes,  powder,  or  sponge,  either  at  the  bottom  of  the 
flask  of  the  apparatus  or  on  the  surface  of  the  plates  of  magne- 
sium. To  render  the  precipitation  complete,  the  liquids  must  be 
kept  in  a  proper  state  of  acidity,  and  the  experiment  prolonged 
till  the  new  plates  of  magnesium  introduced  into  the  liquid  dis- 
solve, whilst  retaining  their  metallic  brilliancy.  To  ascertain 
the  end  of  the  operation,  it  is  well  to  take  out  at  first  a  small 
proportion  of  the  liquid  of  the  flask,  to  put  it  into  a  small  test 
tube,  and  to  introduce  a  well-scoured  ribbon  of  magnesium. 
However  it  may  be,  it  is  always  necessary  to  leave  in  the  flask 
a  small  excess  of  magnesium  before  putting  the  liquid  on  a  filter. 
All  that  is  in  suspension — corroded  plates  of  magnesium,  powder, 
flakes,  or  metallic  sponge — is  washed  on  the  filter  until  the  wash- 
ings show  no  acid  reaction  ;  the  filtered  liquids  should  not  precipi- 
tate on  the  addition  of  hydrosulphuric  acid.  The  filter  being 
dried,  collect  the  deposit  it  contains,  and  analyze  it  in  the 
ordinary  way  to  ascertain  the  metals  precipitated  by  the  mag- 
nesium. 
The  limits  of  an  article  like  this  do  not  admit  of  the  minute 
details  of  the  operations  of  the  experimentalist  and  the  precau- 
tions he  ought  to  take.  These  precautions  and  verifications 
belong  to  all  analyses  appertaining  to  legal  chemistry,  and  every 
operator  imagines,  multiplies,  and  varies  them  according  to  the 
principles  of  the  method  he  employs  and  the  research  he  is  engaged 
upon.  Here  we  bring  forward  merely  a  note  on  the  formation 
of  solid  hydride  of  arsenic,  when  the  liquids  contain  nitric  acid, 
and  the  necessity  in  the  last  case  of  preventing  its  formation  by 
the  addition  of  some  particles  of  pure  sugar  candy,  according  to 
M.  Blondlot's  valuable  recommendation.  Again,  we  content 
ourselves  with  indicating  that  in  a  solution  of  bichloride  of  mer- 
cury the  mercury  is  not  all  precipitated  in  a  metallic  state  by 
the  plates  of  magnesium ;  a  portion  forms  a  deposit  of  protochlo- 
ride. 
