368  NOTES  ON  IODIDE  OF  POTASSIUM,  ETC. 
as  is  known,  is  never  free  from  chloride,  and  the  removal  of 
traces  of  this  salt  is  often  attended  with  much  difficulty,  so  much 
so,  indeed,  that  even  highly  bicarbonated  potash  recrystallized 
often  contains  traces  of  it.  I  suggested  a  process  some  time 
since  for  getting  iodide  of  potassium  free  from  chloride  by  de- 
composing sulphate  of  potash  with  iodide  of  calcium.  With  re- 
ference to  the  moisture  it  will  be  observed  that  cubic  iodide  of 
potassium  nearly  always  contains  free  alkali,  which  secretes  itself 
between  the  interstices  or  plates  or  in  the  cells  that  form  the 
cubes  or  crystals,  and  does  not  entirely  separate  when  the  salt 
is  left  to  drain  for  a  considerable  length  of  time.  A  tempera- 
ture of  212°  also  fails  to  abstract  this  moisture.  On  a  large 
scale  the  crystal  is  submited  to  a  temperature  of  upwards  of  300° 
for  many  hours.  After  this  application,  if  they  are  reduced  to 
powder  and  submitted  to  a  still  higher  temperature,  they  do  not 
lose  weight,  but  if  the  heat  be  gradually  increased  to  the  fusing- 
point,  a  loss  is  at  once  perceptible,  and  the  salt  indicates  the  pre- 
sence of  iodate  of  potash.  If  these  crystals  are  kept  for  a  length 
of  time  or  exposed  to  the  air,  the  deliquescent  alkali  contained 
in  the  interstices  will  attract  sufficient  moisture  to  account  for 
the  percentage  estimated  in  Mr.  Clayton's  analysis  in  most  in- 
stances. When  much  care  is  bestowed  upon  the  preparation  of 
this  salt  the  crystals  possess  great  beauty,  occurring  in  opaque 
or  ivory-white  cubes  or  blocks,  sometimes  in  flat  plates  or  slabs, 
weighing  from  a  few  grains  to  several  ounces  each.  The  opacity, 
transparency,  and  size  of  the  crystal  are  entirely  in  the  hands 
of  the  manufacturer ;  but  under  any  circumstances  the  crystals 
are  never  perfect,  one  or  more  of  the  faces  or  sides  of  the  cube 
being  always  imperfect.  After  operating  upon  large  quantities 
with  unremitting  care  and  adopting  every  contrivance,  I  never 
succeeded  in  producing  a  perfect  cube.  If  the  imperfect  side  of 
one  of  these  cubes  be  placed  upwards  in  a  glass  tube,  and  heat 
applied,  the  form  of  the  crystal  can  be  preserved  and  the  mois- 
ture, if  required,  estimated  {this  side  being  composed  of  the  edges 
of  the  plates  between  which  the  steam  can  escape;)  but  if,  on  the 
other  hand,  the  imperfect  face  of  the  cube  be  placed  downwards 
and  weighed  on  a  platina  or  porcelain  dish,  on  the  very  careful 
application  of  heat  the  cube  will  be  shattered  or  cleaved  into  thin 
