VARIETIES. 
273 
of  chloride  potassium  ;  add  now  to  each  test  tube  a  drop  of 
chlorhydric  acid,  and  then  to  each  one  drop  of  auric  chloride 
solution.  On  now  comparing  the  two  tubes,  particularly  in  the 
direction  of  their  long  axes,  a  yellow  color  will  be  observed  in 
the  tube  containing  the  bromide,  and  made  very  manifest  by 
comparison  with  the  other  tube.  , 
The  following  experiment  shows  the  delicacy  of  the  test  ap- 
plied as  above.  One  centigram  of  potassic  bromide  was  dis- 
solved in  one  thousand  cubic  centimeters  of  water.  Thirty  cen- 
timeters of  this  solution  compared  with  thirty  centimeters  of 
a  very  weak  solution  of  potassic  chloride,  gave  a  decided 
yellow  color.  This  experiment  was  varied  by  dissolving  a  gram 
of  potassic  chloride  in  two  thousand  cubic  centimeters  of  water, 
halving,  and  adding  one  centigram  of  potassic  bromide  to  the 
one  half.  Thirty  centimeters  of  each  of  the  two  solutions  now 
tested,  gave  ample  evidence  of  the  presence  of  bromide. 
The  mixed  chloride  and  bromide  should  be  brought  to  the  state 
of  salts  of  the  alkalis  if  necessary,  by  precipitating  with  argen- 
tic nitrate,  thoroughly  washing,  and  fusing  with  potassic  carbo- 
nate. If  sodic  carbonate  is  used,  the  subsequent  reaction  with 
the  gold  test  is  not  so  decided. 
A  test  for  chloride  in  the  presence  of  bromide,  as  simple  and 
delicate  as  the  above,  is  much  needed.  The  'writer  has  sought 
long  for  it  but  in  vain. 
1607  N.  6th  St.,  Philadelphia,  Pa. 
— American  Journal  of  Science  and  Arts,  March,  1868. 
Mercerising  Cotton. — To  Mr.  Mercer  must  be  attributed  the  discovery 
of  the  peculiar  action  of  caustic,  soda,  and  sulphuric  acid  upon  cotton. 
This  singular  process,  now  called  "  mercerising,"  has  the  effect  of  untwist- 
ing the  normally  twisted  flattened  tubes  of  cotton  filaments  and  convert- 
ing them  into  cylindrical  tubes.  When  colors  are  applied  to  the  cotton  so 
treated,  they  pass  more  readily  through  the  minute  pores  of  the  tubes  and 
are  precipitated  in  denser  layers  in  the  interior  of  the  latter,  whereby 
darker  and  more  permanent  shades  are  produced.    Calicoes  so  treated 
18 
