TESTING  NITRIC  ACID,  ETC.,  FOR  IODINE. 
171 
and  a  rod  of  tin  is  immersed  in  it  until  red  fumes  are  distinctly 
evolved.  The  rod  of  tin  is  then  taken  out,  and  a  small  quantity 
of  sulphuret  of  carbon  is  poured  in  ;  the  mixture  is  shaken  and 
left  quiet  for  a  few  moments.  The  stratum  of  carbon  which  usu- 
ally collects  over  the  acid  appears  of  a  red  color,  unless  the 
amount  of  iodine  in  the  acid  be  too  small.  With  traces  of  iodine 
the  color  of  this  stratum  may  only  be  deep  yellow.  In  this  case 
however,  it  becomes  red,  when  the  sulphuret  of  carbon  is  drawn 
off  and  evaporated  in  a  small  porcelain  capsule  by  blowing  upon 
it. 
To  show  the  sensibility  of  the  test,  1  decigramme  of  iodide  of 
potassium  (or  0-076  grm.  of  iodine)  was  dissolved  in  121  grms. 
of  nitric  acid  free  from  iodine  ;  this  is  nearly  1  :  1600.  In  this 
acid  the  iodine  could  be  very  distinctly  recognized.  This  was 
also  the  case  when  it  had  been  diluted  to  five  times  the  quantity. 
When  diluted  to  ten  volumes,  it  was  no  longer  possible  to  detect 
the  iodine  by  sulphuret  of  carbon.  If  the  limit  of  sensibility  lies 
half  way  between  the  last  two  dilutions,  it  is  1-12, 000th. 
The  tin,  as  may  easily  be  understood,  does  not  act  specifically 
so  that  zinc,  iron,  or  copper  may  be  employed  in  place  of  it ; 
the  action  of  tin  is,  however,  the  most  certain.  It  was  also  clear 
that  it  is  not  the  metal  itself  that  acts  in  this  experiment,  but 
the  lower  grades  of  oxidation  of  nitrogen  produced  by  its  con- 
tact with  the  nitric  acid.  It  was  proved  by  direct  experiments 
that  it  is  nitric  oxide,  the  action  of  which  upon  iodic  acid  was 
already  known.  This,  however,  is  preferable  to  sulphuretted 
hydrogen  and  all  other  reducing  agents,  because  it  cannot  act 
upon  the  nitric  acid  itself,  but  only  quite  directly  upon  the  iodic 
acid.  The  clearest  proof  that  it  is  due  to  the  action  of  nitric 
oxide  is  furnished  by  the  red  fuming  nitric  acid  of  commerce, 
which  needs  only  be  diluted  with  water,  to  enable  iodine  to  be  de- 
tected by  sulphuret  of  carbon. 
Although  one  would  hardly  feel  inclined  to  employ  nitric  ox- 
ide itself  instead  of  tin,  the  author  nevertheless  states  that  the 
latter  has  a  more  certain  action  than  the  former.  Thus,  if  iodine 
be  present  as  chloride  of  iodine,  which,  if  not  always,  may  cer- 
tainly sometimes  be  the  case,  this  is  decomposed  by  tin,  but  not 
by  nitric  oxide. 
By  means  of  the  test  just  described,  the  iodine  in  nitrate  of 
