350 
DETECTION  OF  AKSENIC  BY  COPPER. 
Tannin,  from  very  dilute  solutions  a  precipitate,  caking  to- 
gether on  heating  into  a  resinous  mass. 
Tincture  of  iodine,  a  strong  kermes  colored  precipitate. 
Bromine  water,  a  large  white  precipitate  insoluble  in  water. 
2  drachms  of  colchicin  yield  scarcely  enough  colchicein  for  a 
few  elementary  analyses. — (iV.  Jahrb.  d.  Ph.  xvi.  1—8.) 
J.  M.  M. 
ON   THE  DETECTION  OF   ARSENIC  BY  COPPER, 
By  H.  Reinsch, 
The  following  communication  was  published  in  Erdman's 
Journal  fiir  technische  Chemie,  Ixxxii.  286  :  "  In  the  Smeth- 
urst  poisoning  case,  which  was  lately  disposed  of  in  London, 
the  accused  was  acquitted  in  consequence  of  differing  opinions 
of  the  two  experts,  basing  their  opposing  views  upon  the  em- 
ployment of  one  and  the  same  method,  namely,  Reinsch's  test. 
This,  as  is  well  known,  is  very  delicate  and  reliable,  if  its 
requisite  cautels  are  observed  ;  the  blackish  grey  coating  of  the 
copper,  with  its  metallic  lustre,  is  very  characteristic,  and  still 
two  chemists  had  obtained  different  results  by  this  method. 
From  the  deposition  of  one,  it  appears  that  the  substance  to  be 
tested  for  arsenic  had  been  previously  treated  in  hydrochloric 
acid  solution  with  chlorate  of  potassa,  and  had  given  a  negative 
result  with  Reinsch's  test.  If  we  do  not  admit  the  supposition, 
that  the  arsenic  detected  by  the  other  chemist,  had  been  expelled 
in  the  form  of  chloride,  it  must  be  granted  that  by  this  treat- 
ment the  arsenic  had  been  oxidized  to  arsenic  acid.  Although 
Reinsch,  in  describing  his  method,  speaks  only  of  arsenious 
acid,  it  appears  as  if  the  easy  precipitation  by  copper  from  the 
muriatic  acid  solution  had  been  also  ascribed  to  arsenic  acid, 
and  this  opinion  is  verified  by  the  statements  of  analytical  au- 
thorities (see  Fresenius'  Qualit.  Analyse  10,  Aufl.  §132,  No.  T.) 
A  large  number  of  experiments  have  convinced  me  that  arsenic 
acid  or  arseniates  in  muriatic  or  sulphuric  acid  solution  do  not  pro- 
duce the  grey  metallic  mirror  upon  bright  copper,  either  in  the 
cold  after  standing  for  months,  or  by  boiling  long  continued,  or 
repeated  in  short  or  long  intervals,  unless  the  arsenic  acid  be 
