NOTES  ON  IODIDE  OF  POTASSIUM,  ETC. 
369 
plates  or  small  cubes,  it  is  therefore  evident  if  these  crystals 
were  not  porous  or  one  or  more  of  the  faces  imperfect,  the  salt 
could  not  be*  rendered  anhydrous  without  the  crystals  or  cubes 
being  shattered  to  pieces.  The  fineness  and  beauty  of  the  shell 
that  surrounds  the  sides  of  the  cube  will  not  allow  the  steam 
formed  to  escape  on  the  application  of  heat.  If  one  of  the  plates 
separated  by  heat  or  cleavage  be  examined  under  the  microscope, 
a  saline  body  will  be  observed  to  be  dried  on  the  ivory  surface. 
If  this  surface  be  moistened  with  a  slight  breath  of  steam,  upon 
the  application  of  turmeric  paper,  a  brown  impression  will  be  left 
upon  the  paper,  and  the  whole  of  the  alkali  may  be  thus  removed 
from  both  surfaces,  and  the  thin  plate  or  beeswing  left  will  be 
found  to  be  neutral,  although  if  the  crystals  be  dissolved  in  water 
they  will  scarcely  give  an  alkaline  reaction. 
The  entire  absence  of  bromine  in  all  the  samples  of  iodide  ex- 
amined by  Mr.  Clayton  seems  remarkable,  as  we  find  bromine 
associated  with  iodine  in  much  of  the  fucus  and  ulva  used  in  the 
manufacture  of  kelp ;  and  as  British  manufacturers  do  not  col- 
lect or  separate  the  bromine,  the  question  arises,  what  becomes 
of  it? 
I  had  occasion  to  remark  some  time  ago,  that  when  testing 
bromide  of  potassium  for  the  detection  of  iodide,  starch  failed 
readily  to  indicate  iodide  in  an  alkaline  solution.  This  fact  has 
been  long  since  known  with  regard  to  ozone.  To  be  perfectly- 
sensitive  to  the  influence  of  ozone  the  cubic  iodide  requires  to  be 
exactly  neutralized  by  hydriodic  acid.*  From  a  neutral  solu- 
tion the  salt  then  crystallizes  in  nearly  transparent  or  pale-yel- 
low octohedrons,  with  rhombic  bases,  or  similar  in  geometrical 
form  to  iodine  when  crystallized  from  an  alcoholic  solution  or 
separated  by  sublimation.  I  drew  attention  to  the  value  of  this 
neutral  iodide  some  years  ago  in  photography,  and  it  has  since 
become  a  commercial  article  amongst  photographers.  With  re- 
gard to  the  iodate  often  contained  in  some  of  the  French  or 
foreign  iodide,  I  believe  it  to  be  partly  due  to  the  processes  they 
adopt  differing  from  those  of  the  manufacturers  of  this  country. 
Hitherto  the  French  have  been  enabled  to  produce  iodine  at  a 
*  The  foreign  iodide  does  not  answer  for  this  purpose,  as  it  often  con- 
tains iodate  of  potash. 
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