AVa°rU,ri877.arm'}  Ready  Test  for  Arsenical  Compounds.  127 
tion.  It  is  a  modification  of  Marsh's  test,  a  well  known  method,  and 
is  founded  on  the  circumstance  that  nascent  hydrogen,  in  the  presence 
of  certain  compounds  of  arsenic,  will  give  rise  to  the  formation  of 
arseniuretted  hydrogen  ;  and  thus  very  minute  quantities  of  arsenic, 
under  different  circumstances,  can  be  readily  detected. 
The  modification  used  is  the  employment  of  an  amalgam  of  sodium 
and  mercury  as  a  means  of  generating  the  hydrogen  required  for  the 
test,  and  by  the  use  of  this  substance  do  away  with,  altogether,  the 
necessity  of  any  acid,  and  employ  two  metals  which  are  not  liable  to 
arsenical  contamination.  As  to  sodium,  arsenic  has  never  been  pointed 
out  as  cne  of  its  impurities,  and  as  to  its  presence  in  mercury,  that  is 
a  circumstance  of  very  rare  occurrence  ;  should  it  exist  in  that  metal 
as  an  impurity,  it  can  be  readily  removed  from  it  by  digesting  the 
mercury  in  dilute  nitric  acid,  and  afterwards  well  washing  it  with  water. 
The  amalgam  found  to  answer  best  for  the  test  consists  of  one  part, 
by  weight,  of  sodium  to  eight  or  ten  parts  of  mercury,  and  is  easily 
made  by  heating,  moderately,  in  a  test-tube,  over  a  lamp,  the  mercury, 
and  then  adding  gradually,  in  small  pieces,  the  sodium,  taking  care  to 
keep  the  mouth  of  the  tube  away  from  the  face,  if  unprotected,  lest 
some  of  that  metal,  in  an  ignited  state,  might  be  spurted  out  during 
the  additions  of  the  first  portions. 
The  metals  combine  readily  under  these  circumstances,  forming  an 
alloy  that  is  liquid  whilst  hot,  but  becomes  hard  and  brittle  when  cold. 
The  contents  of  the  tube,  while  still  hot  and  liquid,  are  quickly  poured 
out  on  a  clean  plate,  when  cool  broken  up  for  future  use,  and  imme- 
diately placed  in  a  stoppered  bottle.  The  way  to  employ  this  amal- 
gam is  simply  to  place  the  suspected  matter,  or  solution,  along  with  a 
little  water,  in  the  bottom  of  a  test  glass  or  a  tumbler  ;  then  add  a 
small  bit  of  amalgam,  about  the  size  of  a  grain  of  wheat  ;  and,  lastly, 
place,  without  delay,  on  the  top  of  the  glass  a  piece  of  white  filtering 
paper  or  the  cover  of  a  white  porcelain  crucible,  moistened  with  a  drop 
of  a  dilute  solution  of  nitrate  of  silver,  slightly  acidulated  with  nitric 
acid,  when,  if  arsenic  is  present,  a  dull  black  or  deep  brown  stain  on 
the  paper,  or  a  dark  silvery  one  on  the  porcelain,  will  be  quickly  devel- 
oped in  the  part  moistened,  owing  to  the  silver  of  the  salt  being  reduced 
to  a  metallic  condition  by  the  arseniuretted  hydrogen  thus  evolved. 
The  silver  solution,  found  to  answer  well  for  this  purpose,  is  made 
